Glass fibre reinforced polymeric (GFRP) composites are used in various applications such as aircraft, marine and automobile industries because of their high specific strength, high specific stiffness, light weight, corrosion resistance and non-magnetic properties. They also replace traditional materials in many critical applications. Nonetheless, the machining behaviour of GFRP material is still a challenge to the researchers due to its complicated interaction between fibre and matrix material. On machining, this kind of material suffers by surface delamination, fibre peel up, fibre push up, fibre pull out, fibre fracture, and matrix breaking. With an objective to minimize delamination, an investigation was carried out on Chopped Strand Mat GFRP (CSMat GFRP) material by studying the effects of drill diameter and drill diameter ratio besides spindle speed and feed rate. The experiments were designed by L18 Orthogonal Array and conducted by using standard High Speed Steel tools. The obtained results were analyzed by Signal-to-Noise ratio and Analysis of variance. Based on the Signal-to-Noise ratio analysis, drill diameter of Ø8mm, drill diameter ratio of 0.8, spindle speed of 3000rpm and feed rate of 50mm/min were identified as optimal parameters for drilling CSMat GFRP material with minimum delamination. The ANOVA table results reveal that drill diameter, drill diameter ratio, spindle speed and feed rate have shown statistical significance on delamination.