Objectives: The aim of this study was to find out if gratitude can increase positive affectivity, happiness and optimism in Iranians with low positive affectivity. Method: In this quasi-experimental study, 636 students of three Iranian Universities (Shahed, Vliasr, and University of Sience and Culture) were chosen using pretest-posttest method. Then 96 low positive affect subjects were chosen. Finally, 62 subjects completed satisfaction form and were assigned randomly into seven groups (Gratitude and Neutral Strategies based on the Emmons, Seligman and Verbal-Action patterns, and a Control group), and received separate instructions in specific envelopes. The data required were collected Positive And Negative Affect Scale (PANAS), Oxford Happiness Questionnaire (OHQ), and Life Orientation Test (LOT). Then the data of 50 participants were analyzed with Kruskal-Wallis and Mann-Whitney U statistical methods. Results: The results showed significant increase in positive affects by Seligman and V-A strategies (p=0.20), and increase in happiness by Emmons, Seligman and V-A strategies (p=0.04). Conclusion: The results confirmed the effect of the above three training strategies on the expansion of positive affectivity and happiness.