PS, PP and Nylon plate parts designed with gas channels having five different types of cross-section but with same section area were gas-assisted injection moulded (GAIM). Mechanical properties of GAIM parts were investigated via tensile and bending tests. Effects of part thickness, shape and associated dimensions of gas channels on tensile and bending properties of GAIM parts were examined. It was found that maximum tensile load and ultimate tensile stress show only slight influence from gas channel design and part thickness except Nylon parts which exhibit significant dependence on part thickness due to degree of crystallinity. Gas channel design, introducing additional moment of inertia, results in part structural reinforcement. Part stiffness and maximum bending load basically increases linearly with the moment of inertia and the section modulus of the plate, respectively. Gas channel design attached with top rib (shapes D and E) show the best effect of structural reinforcement. For brittle PS parts, plates with semicircular gas channel (shape A) exhibit maximum flexural strength. PS parts with rectangular gas channel design (shape B) can absorb more bending energy than the other designs. The present study provides part designers with a design guideline for choosing the most effective gas channel design to achieve a specific objective of part structural performance.