Aircraft, aerospace, ships, and rockets are complex engineering systems wi th many thousands of components, glass fiber-polyester composite uses for these engineering applications. These materials are exposed to environmental conditions, studying the strength in the presence of cracks is to developing reliable engineering systems. In this paper, an experimental evaluation of mode I and II intralaminar fracture behavior of chopped glass fiber-polyester composite was conducted and present techniques of specimens‟ preparation, then obtain fracture toughness and strain energy release rate. The laminates were obtained by manual hand lay-up process, cut to obtain ASTM standards of 122 specimens, and exposed to rainwater, saline water, and engine oil for different durations. The test was performed using UTM, critical stress intensity factor (K IC and K IIC) and critical strain energy release rate (G IC and G IIC ) were measured. Through this comparison, mode I fracture toughness were KIC =37.908 MPa.m 0.5 and strain energy release rate 199.55kJ/m 2 , the maximum result in the normal specimen test and was decreased by 73.533%, which was observed in 60 th -days saline water immersion was 10.033 MPa.m 0.5 . While in mode II fracture toughness the maximum value of specimen in the normal test was KIIC =4.378 MPa.m 0.5 and strain energy release rate were 29.4 kJ/m 2 , decreased by 73.8%, which was observed in 60 th -days saline water immersion was 1.147 MPa.m 0.5. The highest degradations of fracture toughness were affected materials immersed into saline water followed by engine oil and rainwater. The results showed that fracture resistance has been found to decrease gradually with the increased duration of environmental exposure; prediction of fracture behavior of composite materials has assumed greater importance for design purposes. Key Words: Glass fiber, Polyester resin, Environmental effect, Compact tension, Asymmetric four-point bending, Intra-laminar failure, Fracture resistance, Fracture toughness