Ｏｂｊｅｃｔｉｖｅ　To compare perioperative atelectasis occurs during transperitoneal robot-assisted laparoscopic radical prostatectomy(T-RALRP) with extraperitoneal robot-assisted laparoscopic radical prostatectomy(E-RALRP) detected by lung ultrasound.　Methods　Forty patients without pulmonary comorbidities who were scheduled for Robot-assisted laparoscopic radical prostatectomy were enrolled in the study and underwent either transperitoneal or extraperitoneal robot-assisted laparoscopic radical prostatectomy. For all the patients, the MAP was recorded. Arterial blood gas was monitored and recorded before anesthesia (T0), 60 min (T1) and 120 min (T2) after pneumoperitoneum and Trendelenberg position was achieved, and after extubation (T3). Before extubation, a total of 240 pairs of lung ultrasound clips were ultimately analyzed to determine the presence and classification of atelectasis in 12 prescribed lung regions.　Results　No significant difference was found for age, body mass index, blood loss, total operation time, PaCO2 and PaO2/FiO2 before anesthesia. T-RALRP had higher MAP (P<0.05). Compared with T0, T-RALRP, PaO2/FiO2 decreased at T1 and T2 after pneumoperitoneum and Trendelenberg position ( P＜0.05, P＜0.01). Whereas, PaO2/FiO2 for E-RALRP decreased at T2(P＜0.05). PaO2/FiO2 values for T-RALRP at T1, T2 and T3 were all lower than value in E-RALRP (P＜0.01, P＜0.05, P＜0.05). PaCO2 showed significant increase at T1 and T2 in both group (P<0.01), but the value of PaCO2 in T-RALRP at T1 and T2 were all higher than the value of E-RALRP (P<0.05). The degrees of 1, 2, 3 of atelectasis detected by lung ultrasound showed significant difference between T-RALRP and E-RALRP (42% vs 28%, 14% vs 5%, 4% vs 2%, P<0.01).　Conclusions　Extraperitoneal robot-assisted laparoscopic radical prostatectomy seems to be less impact on perioperative PaO2/FiO2 and the severity of postoperative atelectasis.