This paper provides a methodology of evaluating performance of tax administration on the basis of available data on effort and achievement variables over a number of years. A case study of sales tax administration in Gujarat State of Indian Union is made to identify the achievement and effort variables. Improvement in total tax potential and realisation of tax revenue and reduction in the level of unrealised revenue are found to be achievement variables. Expenditure on enforcement and training efforts are identified as effort variables. A simultaneous causation model in terms of effort and achievement variables is found to work excellently well to explain past performance. Relatively enforcement expenditure is found to provide more profitable contribution to State revenue in comparison to expenditure on training but several other considerations lead to suggest to work out optimal solutions for both the effort variables in relation to chosen objective of tax administration.