Assessment of driver's mental workload by valid and easy to use instruments is an important issue. Accordingly, this study aims to assess mental workload by the Persian Driving Activity Load Index (DALI) and investigates psychometric properties of this index. The present study was descriptive and of psychometric studies' type. A total of 100 bus drivers were selected by available sampling method and answered to DALI and NASA-TLX questionnaires. For data analysis, factor analysis method, Cronbach's alpha coefficient and Pearson correlation were used by software SPSS version 22 and AMOS version 21. Factor analysis result showed that this questionnaire has good fitness indexes. Significant positive relationships between DALI and NASA-TLX questionnaires (r = 0.30 and p≤0.01) were obtained. Cronbach's alpha was 0.67 for DALI questionnaire, which was acceptable. Findings of the research showed that Persian version of the DALI questionnaire have good psychometric characteristic and it can be used as a valid instrument in this area of studies.