Abstract The 2‐phenyl‐2‐(1‐hydroxyiminoethyl)‐1, 2, 3, 4‐tetrahydroquinazoline (HL) and its NiII complex have been prepared and characterized by spectral method (FT‐IR, NMR (13C and 1H), UV‐vis.), elemental analysis, magnetic susceptibility and thermal analysis (TG, DTA) techniques. The crystal structures of HL and NiII complex were also determined by the single crystal X‐ray diffraction. The HL and NiII complex crystallizes in the monoclinic and triclinic, space groups P21/c and P̄1, respectively. The complex was occurred by the elimination of 1 mole of 2‐aminobenzylamine from the 2 moles of the HL after the ring opening reaction by the NiII attack. Crystallographic study reveal that NiII atom has a square planer geometry being coordinated by four nitrogen atoms of HL. Two thermal processes of the HL and NiII complex can occur in TG and DTA curves.