A large second harmonic generation (SHG) intensity, which was 60 times as large as that of urea was observed in ap‐nitroaniline(PNA)/N‐(p‐nitrophenyl)ethylenediamine (NPEN) system. This SHG intensity was related to the cooling rate for solidification. The optimum cooling rate for SHG intensity was in the range of 2–10 °C/s. X‐ray and thermal analysis done for this new system show that the hydrogen bond and the unique nonplanar structure of NPEN play an important role for SHG and its stability.