Objective To investigate the biomechanical properties of tendon grafts with different diameters used for medial meniscus posterior root repair through biomechanical experiments. This can provide theoretical basis and selection recommendations for the choice of tendon diameter in clinical surgery. Methods Twenty-four fresh porcine knee joint specimens were used to establish standardized medial meniscus posterior root tear models. The specimens were randomly divided into three groups. Twenty-four bovine extensor digitorum tendons were randomly assigned to three groups (eighteen tendons per group) and trimmed to diameters of 3 mm, 5 mm, and 7 mm, respectively. The medial posterior roots were reconstructed with the extensor digitorum tendons.Tendons were pulled through the tunnels and fixed to the tibia with interference screws. Biomechanical tests were then performed to record the ultimate failure load, ultimate displacement, stiffness, and failure modes of all specimens. Results All specimens completed the biomechanical tests successfully.The ultimate failure loads of tendons with diameters of 3 mm, 5 mm, and 7 mm were 153.79±12.37 N, 257.78±26.16 N, and 247.38±21.29 N, respectively; the ultimate displacements were 10.28±1.30 mm, 5.50±1.23 mm, and 5.81±1.58 mm, respectively; and the stiffness values were 16.91±0.76 N/mm, 24.56±2.58 N/mm, and 24.59±2.55 N/mm, respectively. Tendons with diameters of 5 mm and 7 mm exhibited significantly higher ultimate failure load, ultimate displacement, and stiffness compared with those with a diameter of 3 mm(P<0.05). There was no significant difference in ultimate failure load, ultimate displacement, and stiffness between the 5 mm and 7 mm diameter tendons(P<0.05). Two failure modes were observed in the 3 mm tendon group: tendon rupture and meniscal laceration by the tendon. The failure mode in both the 5 mm and 7 mm tendon groups was meniscal laceration by the tendon. Conclusions Tendon diameter can affect the reconstruction strength of the medial meniscus posterior root.In this study, tendons with diameters of 5?mm and 7?mm exhibited superior biomechanical properties for reconstruction compared to those with a diameter of 3?mm.