Background: The purpose of this research is to establish the ability of employees by work ability index (WAI), physical work capacity (PWC), and finding the correlation between them. Establishing the PWC index with attention to WAI values for the purpose of saving in costs and time of PWC measurements is another aim of this project. Methods: The present research is an analytic cross-sectional and one-trail study. The study population consists of 228 randomly selected registered nurses from hospitals in Isfahan (Iran). The WAI and PWC were established through WAI questionnaire and Fax equation and by using ergometer bicycle, respectively. The resulting data were analyzed using SPSS 16 software. Results: Average WAI and PWC among the study population were 38.25±4.4 and 4.45±0.7, respectively. Pearson test results showed no significant correlation between PWC and WAI in different age groups (r=0.3 and P>0.05). Multiple linear regression analysis showed that the variables of age and diagnosed diseases were the most effective factors of WAI (β=0.18 and P>0.05). Pearson test revealed a significant correlation between the number of diagnosed diseases and PWC index in age groups of 40-49 years. Conclusion: Average WAI in this research, like other studies on similar jobs is in the acceptable level of >36. Work ability index and PWC index in different age groups did not show a significant correlation and this suggests that there are essential discrepancies in work ability evaluations made by each index and it is not possible to predict PWC index using WAI values. Given the PWC results and the level of nursing staff's activity (low, medium) the WAI is a suitable instrument to establish the professionals’ abilities. This study revealed that 27.6% of individuals were subject to medium-low work ability risk (WAI<37), which was 1.8 times that of Finland's professional health Institute, which could be caused by shift working and increased working hours.