Obese individuals have reduced quadriceps muscle strength relative to body mass that may increase the rate of progression of knee osteoarthritis (OA). The purpose of this study was to evaluate the effects of isometric exercise training on quadriceps muscle architecture and strength in obese subjects with knee osteoarthritis. Methods: Fortyfour obese male subjects aged 40–65 years diagnosed with knee osteoarthritis were randomly assigned into group A (n=32) and group B (n=12). Group A subjects performed a 12-week isometric exercise program. Group B subjects did not participate in any exercise program and maintained their ordinary activities for the same period. Both groups received the same conventional physical therapy program including hot packs and therapeutic ultrasonic. Muscle thickness, pennation angles and fascicle length of the vastus lateralis (VL) muscle of the affected knee were measured at rest by B-mode ultrasonography. Maximal voluntary isometric knee extension torque (MVIC) of the affected knee was measured using an isokinetic dynamometer. Knee pain and function were evaluated using visual analogue pain scale (VAS) and Western Ontario and McMaster Universities Arthritis Index (WOMAC). All variables were evaluated before and the end of the intervention period for both groups. Results: at the end of the program, group A subjects showed significant improvements compared with group B subjects regarding MVIC and muscle architecture parameters (p<0.05). Also, there was significant improvement in post-test VAS and WOMAC scores in group A subjects compared to group B subjects (p<0.05). Conclusion: A 12-week quadriceps isometric training program improves knee pain and quadriceps muscle strength and architecture in obese subjects with knee OA. These results indicate that isometric training should be regarded as a proper exercise intervention for obese patients with knee OA.