Herein,we demonstrated that poly(ethylene oxide)(PEO),urea and thiourea can crystallize into novel ternary complex with the molar ratio of guest polymer and host small molecule as 3:2,and proved that the ternary complex behaves isomorphism phenomenon by varying the ratio between urea and thiourea for the first time.This observation gives a boost to prepare co-crystals of different small molecules that cannot be obtained by direct mixing without the aid of polymer chains. Herein, we demonstrated that poly (ethylene oxide) (PEO), urea and thiourea can crystallize into novel ternary complex with the molar ratio of guest polymer and host small molecule as 3: 2, and proved that the ternary complex behaves isomorphism phenomenon by varying the ratio between urea and thiourea for the first time. This observation gives a boost to prepare co-crystals of different small molecules that can not be obtained by direct mixing without the aid of polymer chains.