The intrinsic birefringence of Nylon-6 and Nylon-66 were calculated by using the three dimensional coordinates of bond-unit by Bunn et al.Birefringence is a measure of the total molecular orientation of a polymer. For a uniaxially oriented polymer the measured birefringence ΔT may be defined as Where Δ°am is the intrinsic birefringence of the perfectly oriented crystal and Δ°am is the intrinsic birefringence of a perfectly oriented amorphous regions, β the volume fraction of crystalline, and fc and fam the orientation functions characterizing the average orientation of crystalline and amorphous regions, respectively.The intrinsic birefringence of the crystalline and amorphous regions of Nylon-6 were Δ°c=0.096 and Δ°am=0.083, respectively. And, for Nylon-66 the intrinsic birefringence were Δ°c=0.096 and Δ°am=0.077.