Objective The work ability index (WAI) is a common occupational health tool, which has been widely used for indicating individuals’ work ability. However, few studies have been conducted concerning millennial workers. The aim of this study was to investigate factors affecting work ability index in millennial workers who were born in between 1980 and 1995.

Methods The validated Thai version WAI questionnaires were sent to workers at the petrochemical company consists of sections in 21 provinces across all regions in Thailand in February 2016. There are various types of occupations in the company.

Results This cross-sectional study included 1,167 of millennial workers which 684 (58.6%) were men. The mean WAI score was 39.7 (SD = 4.0) in a range of aged 20 to 35. More than half of the subjects were good work ability. The regression technique showed significant association between work ability index and work-life balance. Additionally, demographical characteristics and various factors, which include sex, job tenure, sleep status, work types, and job satisfactions are related to work ability.

Conclusion As various factors, particularly work-life balance, play an important role in work ability. This study suggests related factors, which should be considered for promoting work ability among millennial workers.