Abstract: The aims of this study were to conduct the comparative evaluation of pharmacokinetic of(S)-ornidazole ［(S)-ONZ］ and(S)-ornidazole phosphate disodium ［(S)ONZ-P］ in rats, and to observe the potential chiral transformation to(R)-ornidazole ［(R)-ONZ］ using developed LC-MS/MS method. After single, intravenous administration of 25, 50, 100 mg/kg(S)-ONZ and(S)-ONZ-P ［equal molarity to that of(S)-ONZ］, it was found that there existed linearity between the calculated areas under the curve(AUC_0-∞)and dose. And elimination half-time(t_1/2 )of(S)-ONZ was calculated to be 2. 04-2. 31 h and 2. 02-2. 51 h, respectively. After iv dosing of(S)-ONZ-P, (S)-ONZ-P quickly transferred into(S)-ONZ with the average transformation time of around 1. 57 to 3. 86 min. No significant differences in pharmacokinetics between the two drugs were found in rats. There was no(R)-ONZ detected in the plasma using both(S)-ONZ and(S)-ONZ-P.