[Objectives]  To identify morphological criteria for sexing Trigonorhinus sp., an important wood-boring pest of Caragana liouana Zhao Y. Chang & Yakovlev. [Methods]  The following morphological characteristics of adult Trigonorhinus sp. were measured; body length, body width and length of the 6th abdominal plate, as well as whether the wing sheath completely covered the end of the abdomen. [Results]  1) The end of the abdomen of females is concave and bifid in the center, whereas that of males is bluntly rounded. 2) The 6th abdominal plate of females is significantly longer (0.48± 0.01) mm than the 5th plate of males which was significantly shorter [(0.30±0.01) mm]. The proportion of body length comprised by the 6th abdominal plate [(0.16±0.01) mm] is greater in females than in males [(0.11±0.02) mm]. 3) Wing sheaths completely cover the end of abdomen in females, but not in males. [Conclusion]  Although is difficult to sex adult Trigonorhinus sp. on the basis of body size alone, they can be readily sexed by the length of the 6th belly plate. However, definitive sexing requires examination of the external genitalia.