OBJECTIVES: This study aims to evaluate the effects of physical therapy modalities five days a week versus twice a week with a three-day interval on pain, depression, and functional disability in patients with chronic mechanical low back pain. PATIENTS AND METHODS: Between June 2018 and November 2018, a total of 60 patients (39 females, 21 males; mean age 41.0±6.8 years; range, 25 to 55 years) with chronic mechanical low back pain were included. The patients were divided into two groups: Group 1 (n=30) received physical therapy five days a week for a total of 15 sessions, while Group 2 (n=30) received physical therapy twice a week with a three-day interval for a total of six sessions. Per protocol, all patients were applied hot pack for 20 min, followed by conventional transcutaneous electrical nerve stimulation (TENS) for 20 min and therapeutic ultrasound (US) for 10 min in each session. All patients were given exercise training and were instructed to do twice a day up to four weeks after the completion of physical therapy sessions. The severity of low back pain was evaluated using the visual analog scale (VAS), the severity of depression using the Beck Depression Inventory (BDI), and functional disability using the Oswestry Disability Index (ODI). All patients were evaluated before and after physical therapy and at four weeks. The results were compared between the groups. RESULTS: There was no statistically significant difference in the age, gender, body mass index, occupation, VAS, BDI, and ODI scores at the time of randomization (p>0.05). Intra-group analysis showed a significant improvement in all parameters after physical therapy compared to pre-treatment values and at four weeks compared to the post-treatment values (p<0.001). The differences of changes were used for the inter- group analysis which revealed a difference of change in favor of improvement was significantly higher in Group 2 than Group 1 (p<0.001). There was no significant difference between the post-treatment values and values at four weeks (p>0.05). CONCLUSION: Our study results suggest that both physical therapy modalities applied five days week and twice a week are effective in pain, depression, and functional disability. However, physical therapy protocol applied twice a week yields more improvement, indicating that it is more advantageous with less cost and lost workdays.