Objective: To observe the expression of myosin heavy chain (MHC) and phosphorylated 20 kD myosin light chain (P-MLC20) in the smooth muscle of the common bile duct (CBD) from children with pancreaticobiliary maljunction (PBM). Methods: The CBD specimens from 16 PBM pediatric patients were compared with 16 CBD specimens from cadavers of neonates serving as control. The MHC and P-MLC20 protein expressions in the two tissues were examined and analyzed by image analysis system after immunohistochemical staining, and the P-MLC20 protein expression in the two tissues were also determined by Western blot analysis. Results: The immunohistochemical semi-quantitative analysis for the MHC and P-MLC20 protein expressions showed there were no differences in mean optical density (MOD) for both proteins between the two groups (both P>0.05), but the mean labeling intensity (MLI) and minimum qualifying scores (MQS) for both proteins in PBM group were all significantly higher than those in control group (all P<0.05). Western blot results showed that the expression level of P-MLC20 protein in PBM group was significantly increased compared with control group (P<0.05). Conclusion: The MHC and P-MLC20 expressions are increased in CBD smooth muscle of PBM children, which may probably lead to the compensatorily enhanced contractile ability of their CBD smooth muscle.