The present work focuses on primary objectives to analyse the dimensional stability of machined samples through kerf width and taper angle. Further to optimize the process parameters to obtain optimum results. RSM-BBD approach has been used to design the experiments. We prepared Mg based metal matrix hybrid composite samples using stir casting process. Prepared samples are machined on CNC wire cut EDM. Matrix of Mg alloy AZ91 (94%) and reinforcement of SiC (4%) and Al2O3 (2%) powder is selected. Machined samples are observed for kerf width and taper angle calculations followed by the optimization of process parameters to obtain optimum kerf width and taper angle. Optical microscope is used to measure the kerf width with 10x magnification. Top kerf width (Tkw), bottom kerf width (Bkw) and taper angle have been calculated and parametric plot is shown. Results obtained shows that pulse on time and current is most significant factor in obtaining the optimum values of output responses. Optimum values of top kerf width and bottom kerf width are obtained at Ton = 18µs, Toff = 5µs, I = 3A & WS = 10.4m/s and Ton = 18µs, Toff = 9µs, I = 3A & WS = 3.12m/s respectively. Optimum value of taper angle is obtained at Ton = 30µs, Toff = 7µs, I = 3A & WS = 10.4m/s.