Abstract: AimTo study the structure and crystal forms of chlorobenzylidine. MethodsKarl Fischer titrimetry, FTIR, thermal analysis, single and powder X-ray diffraction were used for the studies of the structure of chlorobenzylidine and for the identification of two forms of chlorobenzylidine. Results Chlorobenzylidine and its diastereoisomer have been studied in this article. They can be distinguished by their different melting points. Two crystal forms of chlorobenzylidine (form A and form B) have also been detected and studied. Form A was studied by single-crystal X-ray diffraction, it crystallized in the triclinic system, space group P-1, with two formula units per cell, is monohydrate. Karl Fischer titrimetry, FTIR, thermal analysis and powder X-ray diffraction were used for identification of the two forms.Conclusion The studies of structure and crystal forms of chlorobenzylidine are very useful for the clinical research and the selection of recrystallization process.