Experimental studies of successive structural phase transitions of KNiCl₃ and related crystals (KNiCl₃-family crystals) RbMnBr₃, TlFeCl₃, TlCoCl₃, RbFeBr₃, RbCoBr₃ and RbVBr₃ with dielectric and diffraction methods were summarized. Their prototype structure is the CsNiCl₃-type structure, which is known to contain NiCl₃ linear-chain substructure. Some phases of GroupsI(KNiCl₃, RbMnBr₃, TlFeCl₃ and TlCoCl₃) , II(RbFeBr₃) and III(RbCoBr₃ and RbVBr₃) crystals were shown to be ferroelectric with spontaneous polarization parallel to the chain (c-axis). Transition points and space group symmetries of each phase were listed. It was discussed that those structural phase transitions can be attributed to competition between the distortion of NiCl₆ octahedra which are directly connected to the spontaneous polarization and that of KCl₁₂ polyhedra which might be closely related to the softening of vibrational K₄-mode. The reasons why some crystals with the CsNiCl₃-type structure (such as RbNiBr₃) does not undergo such structural phase transitions and why the spontaneous polarization of Group I KNiCl₃-family crystals disappears in phase V remain unsolved.