Optical second-harmonic generation (SHG) of liquid crystalline main-chain polymers, which are the copolymers of 2-hydroxy-6-naphthoic acid with 4-hydroxybenzoic acid, and their application for SHG device have been studied. Varieties of copolyesters which have various composition-ratios were prepared. The SHG activities of each sample were evaluated by both the powder and Marker fringe methods. It was found that the double orientation processing was effective to obtain the sample of high SHG activity. At present, the effective nonlinear optical coefficient d(eff) of the sample with the composition PHB/HNA (60/40), which was prepared to be a plane-plane oriented (double orientation) film of thickness 10-30 mu m, is obtained to be 10 times that of quartz d(11) value (0.50pm/V).