The aim of this research was to reduce shrinkage porosity and hot spot defects in aluminum alloy pulley casting products by using grey based Taguchi method and simulation. The casting products are manufactured by conventional or trial and error method which is subjected to many casting defects and these defects lead to poor product quality and less productivity in the factory. The objective of this study was to optimized green sand casting process parameters by using grey based Taguchi method and simulation to reduce casting defects in aluminum alloy casting pulley casting. The maximum and minimum casting defects were determined from the simulation results and the optimum parameters were selected based on grey based Taguchi method. The confirmation test results were done by simulation using ProCast software. Analysis of variance (ANOVA) was used to determine the significant and insignificant parameters. The selected parameters were spare, runner, in-gate and riser dimensions and the defects were reduced shrinkage porosity and hot spot defects. The selected optimum parameters of sand casting process were sprue inlet diameter 28.59 mm, sprue outlet diameter 19.17 mm, sprue height 140mm, runner length 50.48mm, runner width 25.24 mm, runner depth 28.96mm, in gate length 28.96 mm, in gate width 28.96 mm, in gate depth 28. 96 mm, riser top diameter 60.94 mm, riser bottom diameter 51.52 mm and riser height 35.47 mm. the significant parameters are sprue outer diameter, runner breadth and in-gate breadth. In current sand casting processes the maximum shrinkage porosity and maximum hotspot defects are 68.22% and 543.1 sec. respectively. But in the optimized casting process parameters the maximum shrinkage porosity was 46.07% and maximum hot spot was 415.0 sec. The improvement in reducing shrinkage porosity and hot spot defects was 32.47% and 23.59% respectively. Therefore, optimization of sand casting parameters are reducing casting defects and improve product quality since optimization is continues and never ending processes. Keywords: Casting parameters, Levels, Simulation, Grey based Taguchi method, Shrinkage porosity defects, Hot spot defects, Analysis of variance (ANOVA).