Objective To study and compare biomechanical properties of a newly developed magnesium AZ31B alloy intramedullary nail (AZ31B) with that of imported Poly-L-lactic acid intramedullary nail (PLLA) and pure titanium rib plate (TPRP), so as to provide scientific evidences for better internal fixation in clinical operation. Methods Forty fresh adult ribs were used and divided into 4 groups randomly. Three groups were made lateral rib fracture in midaxillary line and fixed by AZ31B, PLLA and TPRP, respectively, while the group with normal ribs was used as control. Biomechanical properties of specimens in each group were measured and tested using experimental stress analysis. Results (1) Three-point bending strength of internal fixation with AZ31B was close to that of control group (P>0.05), but significantly different to that of TPRP group and PTRP group (P<0.05); (2) No significant differences in torsional strength were found between AZ31B group, PTRP group and control group (P>0.05), and the torsional strength of both AZ31B and PTRP was superior to that of PLLA (P<0.05). Conclusions The internal fixation with AZ31B is an ideal mode for treating rib fracture since AZ31B has larger flexural strength than PLLA and TPRP, and its torsional strength was close to PTRP and normal ribs. This study provides some support for future research on biomechanical properties of AZ31B.