Aims: The aim of the study to find out the effectiveness of Shortwave Diathermy Treatment on Muscle Power in Patients with Chronic Low Back Pain. Materials and methods: This was an interventional study conducted in the Department of Physical Medicine and Rehabilitation, Anugrah Narayan Magadh Medical College and Hospital, Gaya, Bihar, India for 1 year. 150 patients were randomly divided into 3 groups. There were 50 patients in each group. In the first group, placebo shortwave diathermy was applied while the device was closed. Continuous shortwave diathermy (27.12 MHz frequency and 11.06 m wavelength, 200 watts) was applied in the second group, while the third group received pulsed shortwave diathermy (27.12 MHz frequency and 11.06 m wavelength, 200 watts, 0.3 ms pause). The isokinetic muscle strength measurements of patients were performed using a Cybex isokinetic system (Cybex-Norm) before and 3 months after the treatment. Results: Out of 150 patients 35 was males and 115 females in our study, mean age was 50.68 ± 5.89 years. Males were more frequent in the Group 3 than other groups (p=0.049).When we evaluated isometric muscle strength, flexion strength was significantly higher after treatment than before for Group 1 (p=0.001). On the other hand there was no significant difference between measurements regarding extension (p=0.352) and rotation (p=0.072) strength. There was no significant difference between before and after treatment results regarding flexion, extension and rotation muscle strength for other groups. The increase in isometric flexion strength was significantly higher for Group 1 than Group 3 (p=0.019).There was no significant difference between our groups regarding isometric extension strength (p=0.624) and isometric rotation strength (p=0.059).When we evaluated isokinetic muscle strength at 60°/sec angular speed, flexion strength (p=0.013) and extension strength (p=0.005) were significantly higher after treatment than before treatment for Group 1, while there was no significant difference between measurements regarding rotation strength (p=0.411). In Group 2, flexion strength (p=0.006) and extension strength (p=0.031) were significantly higher after treatment than before treatment, while there was no significant difference between measurements regarding rotation strength (p=0.597). In Group 3, there was no significant difference between before and after treatment results regarding flexion, extension and rotation muscle strength. Conclusion: There are no significant differences between exercise therapy alone and exercise therapy in combination with diathermy (either continuous or pulsed) in terms of their effects on lumbar muscle strength.