Objectives: In old age depression is common problem. Its rate at elderly residing at nursing homes is 30-50%. Several types of medications are used to treat depression. However because elderly use many drugs due to their numerous problems, non pharmacological methods such as pet therapy can be considered. So the present study intended to assess the effectiveness of pet therapy on elderly depression.  Methods & Materials: A quasi-experimental study was conducted with 60 elderly living at nursing homes. The elderly were randomly placed at two groups: intervention group (N:29) and control group (N:31). In both groups Geriatric Depression Scale (GDS) and a questionnaire for demographic characteristics were used to collect data. Then research intervention including animal care program over a six-week period was performed. And the control group did not receive this intervention. Both groups were assessed by The Geriatric Depression Scale after the intervention period. Pretest score and posttest score were compared. To analyze the collected data Chi-square and independent t-test were used and the dependant t-test was applied to analyze the variables. Results: Findings of this study showed that depression of elderly of intervention group decreased from 8/55 before intervention to 4.79 after intervention (P<0.001). But no significant difference was observed in the control group. The comparison of depression scores mean in both groups before intervention shows no significant statistical difference (P=0.531) while the mean after the intervention in control group was significantly higher (P=0.001).  Conclusion: Considering the results of this study, it can be suggested that pet therapy has positive effects on reduction of elderly depression. Therefore, non-invasion interventions such as keeping pet by nurses can improve elderly depression and can cause their active participation. It is strongly recommended to use this intervention widely at nursing homes. In addition, this study provided a new field for research and nursing services.