Multi-wire sawing process with slurry has been popularly adopted for wafer slicing of silicon substrates for solar cells. This paper is to investigate the chip size estimation for effective blending ratio of mixing slurry of wire sawing. Different combination of slurry has been studied with microscopic pictures of abrasive grits by SEM and distribution of particle size of slurry. The chip size can be estimated by the developed method and then the TTV of sliced substrates is used to evaluate the blending ratio of slurry for cost efficiency of wire sawing. Experimental results indicate that the slurry with certain ratio of reborn grits increases the TTV value of sliced substrates. As the weight percentage of silicon chips is above 6 % for the blending ratio of 50% reborn abrasive grits after sawing run 2, the slurry is suggested to be changed to new slurry for wire sawing. Under this developed rule, the average TTV can be maintained under 0.016 mm for nominal thickness of 200 m silicon substrates. Results of this paper can be used to estimate the feasible blending ratio for maintaining the TTV under the desired specifications of silicon substrates for cost efficiency.