The main purpose of this paper is to investigate the mediating role of reciprocity in understanding knowledge contributions in one of the biggest open source communities, GitHub. In 2014, We are extracting the massive open data that offered by GitHub. 400 top open source project that has most pull-request in Javascript categories and not belongs to any organization were selected as sample for the study. We analyzed the data using SmartPLS 3.0. In this preliminary study, all of hypothesis in our research model are significant. The study revealed that reciprocity acted as suppressor variable between member size and quantity of knowledge contribution.