The second harmonic generation (SHG) of the liquid crystalline main-chain type copolymers of 2-hydroxy-6-naphthoic acid (HNA) with 4-hydroxy-benzoic acid (PHB), in various composition ratios were investigated. The SHG of the films was greatly affected not only by the polymerization conditions but also by the processing conditions of the films. The nonlinear optical coefficient d(exp) of the electrically oriented sample is 3-5 times larger than that of the mechanically oriented sample (5pm/V). The relationship between molecular compositions and the hyperpolarizability (beta) examined by the molecular orbital method clearly indicated that the high SHG of the main-chain polymer mainly originates from the additivity of beta value of individual unit due to head to tail connection.