[Purpose] To determine the intra-rater and inter-devices reliability of isometric knee-extensor muscle-strength-measurement. [Participants and Methods] The participants were 77 university students (50 males; 27 females) who underwent isometric knee-extension muscle-strength measurement twice with a belt-stabilized handheld dynamometer and isokinetic dynamometer. The intra-rater and inter-devices reliability was ascertained from measurements that were performed with the participant seated on the edge of the bed, the toes of the contralateral nonexamined side off the floor, and the trunk supported by the upper limb. [Results] In the overall, male, and female cohorts, the 95% confidence intervals of intra-rater reliabilities (1,1) in the belt-stabilized handheld and isokinetic dynamometers were 0.96-0.98, 0.92-0.97, and 0.81-0.96, 0.82-0.92, 0.73-0.90, and 0.78-0.95, respectively, and the inter-device 95% confidence intervals were 0.52-0.77, 0.28-0.69, and -0.03 to 0.63, respectively. Compared to the belt-stabilized handheld dynamometer group, the isokinetic dynamometer group had higher knee-extension muscle strength. [Conclusion] The intra-rater reliability was rated good-to-excellent and moderate-to-excellent for the belt-stabilized handheld and isokinetic dynamometers, respectively. The inter-devices reliability was poor, and isometric knee-extension muscle strength in the isokinetic dynamometer group was higher than that in the belt-stabilized handheld dynamometer group.