Abstract: Ohmefentanyl,N-1-(2-hydroxyl-2 phenyl)-3-methyl-piperidyl-N-phenylpropanamide,is a new and potent analgesic agent.Two stereoisomers :cis-(3R,4S,2′R)-ohmefentanyl(Ⅰ) and trans-( 3R,4R,2′S)-ohmefentanyl(Ⅱ),were selected for X-Ravcrystallographic study.The two isomers were shown tO have an sp³N(1)and an sp²N(7) and thepiperidine ring has a chair conformation,The cis-isomerⅠhas an axial 3-methyl with an equatorial 4-N-phenylpropanamide group, while the two groups of the trans-isomer Ⅱ are all equatorial. In Ⅰmolecule , the plane formed by the 4-propanamide group and atom C(4)is almost perpendicular to the4-N-phenyl ring plane; in Ⅱ, the dihedral angle of the two planes is about 100°. Intramolecularhydrogen bond of O(1)一H…N(1) occurs in both the two isomers and isomer Ⅱ is also involved inintermolecular hydrogen bond O(1)一H(A)…O(2)(B).