Background Nicardipine, a calcium channel blocker, is used to treat hypertension in pregnancy or preterm labor. The current study was conducted to investigate the relaxant effects of nicardipine on the isolated uterine smooth muscle of the pregnant rat. Methods We obtained uterine smooth muscle strips from pregnant female SD rats. After uterine contraction with oxytocin 10 mU/ml, we added nicardipine (10 −12 to 10 −8 M) accumulatively every 20 min. We recorded active tension and frequency of contraction, and calculated EC 5 (effective concentration of 5% reduction), EC 25 , EC 50 , EC 75 , and EC 95 of active tension and frequency of contraction using a probit model. Results Nicardipine (10 −12 to 10 −8 M) decreased active tension and frequency of contraction in a concentration-dependent manner. The EC 50 and EC 95 of nicardipine in the inhibition of active tension of the uterine smooth muscle were 2.41 × 10 −10 M and 3.06 × 10 −7 M, respectively. The EC 50 and EC 95 of nicardipine in the inhibition of frequency of contraction of the uterine smooth muscle were 9.04 × 10 −11 and 4.18 × 10 −7 M, respectively. Conclusions Nicardipine relaxed and decreased the frequency of contraction of the uterine smooth muscle in a concentration-dependent pattern. It might be possible to adjust the clinical dosage of nicardipine in the obstetric field based on our results, but further clinical studies are needed to confirm them.