Background and Objective: In recent decades studies have shown that the main cause of accidents is unsafe acts. Installation of safety signs is a complementary corrective action for unsafe act control. Therefore, the objective of the present study was to investgate the effects of safety signs on the modification of unsafe behaviours. Materials and Methods: This interventional study was carried out in one of the turbine manufacturing industries in Iran, based on safety behaviour sampling technique. The unsafe acts was first determined and recorded, then 71 standard safety signs was installed based on (ANSI -Z535.2 & 3-2007) and 3 and 6 months after that the behaviours were evaluated. The behaviours before and after using the safety signs were compared. Results: The frequency of unsafe behaviors was 41.7% before the intervention. The most frequent unsafe behaviours were not or incorrect use of personal protective equipments, taking improper postures and unsafe work at height. There and 6 months after the intervention, unsafe behaviours reduced to 27.3% and 31.3% respectively (p<0.005). Conclusion: The findings showed that safety signs have a statistically significant effect on the reduction of unsafe behaviours. However, this effectiveness will be reducing over the time.