Background and purpose: This research was conducted with the aim of investigating the mediating role of organizational trust in the relationship between fun at work and informal learning in public organizations in Shiraz. Method: The current research is practical in terms of its purpose, the research is survey-analytical and based on covariance matrix analysis using structural equation modeling (SEM). Using stratified random sampling method, 171 managers and vice presidents of different organizations were selected as samples. Findings: In examining the relationships between variables, despite the mediating role, direct, indirect and total effects should be examined. If the relationship between fun at work and trust or the relationship between trust and informal learning is not significant, then the mediation will be complete and if all relationships are significant, then there will be a partial mediation, which according to the obtained value, organizational trust has a partial mediation in the relationship between fun at work and informal learning. Results: The results of the research show that all the direct and indirect effects between the variables, that is the effect of fun at work on organizational trust, fun at work on informal learning and organizational trust on informal learning were all confirmed. It should be noted that the strongest effect coefficient is related to the effect coefficient of fun at work on organizational trust, and the weakest coefficient is related to the coefficient of organizational trust on informal learning.