Abstract： Objective To study the relaxant effect mechanism of an estrone derivate EA303 on isolate colonic smooth muscle of rabbits. Methods Preparations of the isolated colonic smooth muscle of rabbits were prepared. The effect of EA303 on potassium channel, β receptor and prostaglandin were studied by observing the difference of relaxant dose-effect curves of EA303 on preparations pre-contracted with BaCl3, High K+ solution and Acetylcholine chloride (ACh) in the absence or presence incubation with glibenclamide (10 μM), propranolol (0.1 μM) and Indometacin (10 μM). Results The relaxant effect of EA303 on contraction caused by BaCl2 and High K+ solution were weakened by glibenclamide inhibiting the opening of K+ channel while the relaxant effect of that on contraction caused by ACh was strengthened, after adding propranolol inhabiting β receptor, EA303 attenuated the relaxant action on contraction caused by BaCl2. EA303 had some relaxant impact on contraction induced by High K+ solution after adding indometacin inhabiting the synthesis the prostaglandin (PG). Conclusions The relaxant effect of EA303 on isolated colonic smooth muscle of rabbits may be related with PG synthesis enzyme, potassium channel and β receptor.