Introduction: Firefighting is one of the occupations that requires high physical and aerobic power due to the nature of the job and the physical demands of this profession. Therefore, the present study aimed to investigate the relationship among workability index (WAI), mental workload, musculoskeletal disorders (MSDs) and their effective factors in firefighters. Methods: This descriptive-analytical study was carried out on 250 firefighters in Tehran in 2019. Data were collected using WAI, NASA-TLX and Body Discomfort Chart (BDC). Data analysis was carried out using descriptive statistics, linear regression, Pearson, and Spearman correlations Results: The mean (SD) of total mental load was 70. 07 (6. 58). Also, the mean (standard deviation) of the total WAI score was 38. 85 (1. 17). Physical workload dimension had the highest value among mental workload dimensions 93. 49 (8. 93). The results showed that WAI is increased by 0. 14 by augmentation of one unit at the academic level of firefighters (p = 0. 03). The most common discomfort was reported in the lower (50 people (20%)) and upper (42 people (16. 8%)) regions of the back. The total WAI relationship with total mental workload was positive and insignificant (p>0. 05). There was a significant and reverse association between WAI and discomfort in the wrist, leg, and ankle. Conclusion: Despite the low rate of MSDs, even mild symptoms of musculoskeletal pain should be considered. Given the high level of mental workload, improving physical, psychosocial and social working conditions is important in increasing the ability of employees and augmenting the ability of their occupational activity.