Farmers' knowledge of IPM has been considered, as prerequisites to IPM adoption. In Iran the most prevalent extension method for training farmers in different fields like IPM is extension workshops. The main purpose of this study was to investigate workshop effectiveness to change farmer's knowledge of IPM through assessing and comparing farmer's knowledge of IPM. A survey was conducted among 90 farmers in Karaj City in 2007--08 that included 30 workshop participants, 30 farmers who had exposed to workshop participants, and 30 randomly selected farmers, which were outside of this community. A questionnaire including open and close ended questions was designed. Data was collected through personal structured interviews with respondents at their farms. The study found that workshop participants (10.38) had significantly higher knowledge scores than their non workshop counterparts (5.90). Data also showed very little diffusion of workshop-acquired knowledge from workshop participants to other community members.