Tax is a major and indispensable source of government revenue that sustains nations across the world. It is a compulsory and non refundable payment to the government by citizens and residents. Tax revenue from the non-oil sector in Nigeria, has been relatively low over the years and insufficient for any meaningful development thus the need for an efficient tax system to adequately generate tax revenue for the government. The need to evaluate the performance of the Federal Inland Revenue Service (FIRS) cannot be overemphasized in view of the current challenges of dwindling oil revenue, infrastructural deficit and other socio-economic challenges prevalent in the country. 

It is in this vein that the research was undertaken to examine the effect of -quality of FIRS staff, strategies used by FIRS, the income level of tax payers and the environmental factors, on the performance of FIRS in tax revenue generation in the North West Geo-Political Zone of Nigeria from 2008-2013. Primary and secondary data sources were used for the study. The primary data was obtained through administration of 348 questionnaires and interviews conducted with thirteen management staff and 22 focused group members while the secondary data was obtained from FIRS Publications, electronic facilities and other related materials. These were analyzed using tables, simple percentages, bar charts. Pearson's Product Moment Correlation Coefficient (PPMCC) test was applied in testing hypotheses level of significance between each of the variables; quality of FIRS staff, tax strategies, impact of environmental challenges, income level of tax payers and the performance of FIRS in tax revenue generation the North West Geo-Political Zone of Nigeria. 

The PPMCC result revealed positive correlations coefficient of r = 0.799, r = 0 827, and r =854 respectively. The Coefficient of Determination was used to further explain the strength of the relationships in the variables of the research hypotheses and the results affirmed a strong positive correlation which rejected the null hypotheses and accepted the alternate hypotheses of the study. The findings of the study showed a relatively poor performance by FIRS in tax revenue generation in the North West Geo-Political Zone of Nigeria due to insufficient staff, tax strategies, environmental challenges and low income levels of the tax payers. 

There is also lack of efficient tax coverage by FIRS and a large informal sector not captured in the tax net of FIRS, tax evasion and tax avoidance. Poverty, poor tax records, poor tax culture, and low compliance level of tax payers, were also identified as hindrances for an enhanced tax revenue yield alongside deficiencies in government's transparency and accountability, infrastructural deficit and low socio-economic activities in the North West Geo-Political Zone of Nigeria. 

The study recommended that all issues of FIRS staff inadequacies, remuneration, training and welfare be adequately addressed in order to enhance its efficiency and performance in tax revenue generation in the zone. Infrastructural and agricultural development and complete automation of FIRS tax system and its tax administration machineries by the government will greatly improve the quality of the tax records and ensure an efficient and corrupt-free tax administration. There is also the need to enforce tax revenue accountability and transparency of government officials in the management of tax revenue, tax education, expansion of tax revenue base, and good governance, will ginger people's interest towards increased economic activities and voluntary tax compliance. These will greatly improve the performance of FIRS in tax revenue generation in the North West Geo-Political Zone of Nigeria and Nigeria at large.