In this paper, an analysis of test time by CBET (which is an acronym for Combination of BIST and External Test) test approach is presented. The analysis validates that CBET test approach can achieve shorter testing time than both external test and BIST in many situations. An efficient test time minimization algorithm for CBET-based LSIs is also proposed. It uses several characteristics of CBET test approach derived by the analysis to reduce computation time to find the optimum test sets. The algorithm helps designers to save their precious design time.