Ineffective tax audit has been considered as one of the many reasons behind non-compliance to tax laws and revenue loss. Therefore, the present study evaluated the effect of audit case selection on tax audit effectiveness in South Rift Valley Region, Kenya. The study was premised on Hoffman’s Tax Planning Theory. The study used descriptive research design targeting a population of 80 tax officers in South Rift Valley Region; therefore, it adopted a census method. Pre-tested questionnaires were used to collect data. Data was analyzed using descriptive statistics and inferential statistics. The findings revealed that audit case selection (β = 0.487, p <0.05) was directly related to tax audit effectivenessin South Rift Valley Region. The study recommends that there is need to improve on the audit case selection procedures that can be applied on both large taxpayers in formal sector and small taxpayers in informal sector.