Abstract: Objective To compare the pharmacokinetics of cisatracurium between normal weight patients and morbidly obese patients. Methods Twelve obese ASA I-II patients (BMI≥35 kg/m2) undergoing laparoscopic Roux-en-Y gastric bypass and 12 normal weight ASA I-II patients (BMI of 18.5-24 kg/m2) undergoing laparoscopic surgery were enrolled. The obese patients were given a cisatracurium dose of 0.15 mg/kg according to the fat-free mass (FFM), and the non-obese patients received a dose of 0.15 mg/kg according to the total body weight. Plasma concentrations of cisatracurium was monitored in the patients with high-performance liquid chromatography (HPLC) before anesthetic induction and at 1, 2, 4, 6, 8, 10, 12, 15, and 20 min after cisatracurium administration and the pharmacokinetic parameters were computed. SBP, DBP, HR, MAP, SpO2 and PetCO2 were recorded before anesthetic induction (T0) and at 1 min (T1), 2 min (T2), 4 min (T3) after cisatracurium administration. Results Compared with those measured at T0, SBP, DBP and MAP in the 2 groups were significantly decreased at the time points of T1-3 (P<0.05). Compared with the non-obese patients, the obese patients showed significantly increased Hct level (P< 0.05). The total clearance, apparent volume of distribution, and distribution and elimination half-life of the drug were similar between the 2 groups (P>0.05). The plasma concentration of cisatracurium at T1-2 was significantly decreased in the obese patients compared with that in the non-obese patients (P<0.05). Conclusion Cisatracurium doses according to fat-free mass is clinically reasonable for inducing anesthesia in morbidly obese patients, but due to a prolonged muscle relax onset time, the timing of tracheal intubation should be delayed by 1-2 min.