Background Ropivacaine and levobupivacaine have been introduced into obstetric analgesic practice with the proposed advantages of causing less motor block and toxicity compared with bupivacaine. However, it is still controversial whether both anesthetics are associated with any clinical benefit relative to bupivacaine for labor analgesia. This study aimed to compare the analgesic efficacy, motor block and side effects of bupivacaine, ropivacaine and levobupivacaine at lower concentrations for patient-controlled epidural labor analgesia. Methods Four hundred and fifty nulliparous parturients were enrolled in this randomized clinical trial. A concentration of 0.05%, 0.075%, 0.1%, 0.125% or 0.15% of either bupivacaine (Group B), ropivacaine (Group R) or levobupivacaine (Group L) with sufentanil 0.5 μg/ml was epidurally administered by patient-controlled analgesia mode. Effective analgesia was defined as a visual analogue scale score was ≤30 mm. The relative median potency for each local anesthetic was calculated using a probit regression model. Parturients demographics, sensory and motor blockade, obstetric data, maternal side effects, hourly volumes of local anesthetic used, and others were also noted. Results There were no significant differences among groups in the numbers of effective analgesia, pain scores, hourly local anesthetic amount used, sensory and motor blockade, labor duration and mode of delivery, side effects and maternal satisfaction (P >0.05). The relative median potency was bupivacaine/ropivacaine: 0.828 (0.602-1.091), bupivacaine/levobupivacaine: 0.845 (0.617-1.12), ropivacaine/levobupivacaine: 1.021 (0.774-1.354), respectively. However, a significantly less number of effective analgesia and higher hourly local anesthetic use were observed in the concentration of 0.05% than those of ≥0.1% within each group (P