Baseline data represent one of the important stages of Environmental Impact Assessment (EIA) procedure that describes the existing environment of the study area and surrounding areas in enough detail to allow the environmental impacts of the proposed area to be accurately and adequately assessed, and future changes and effects can be measured. Baseline data may be inaccurate, difficult to obtain or non-existent in Iran as a developing country, and it involves huge and diverse environmental data of a spatial nature in the EIA process. Therefore, a useful and effective geographic information system (GIS) approach is developed to integrate geodatabase by acquiring and handling environmental data from different sources related to the proposed project area for the EIA procedure. Based on this approach, a comprehensive geodatabase for baseline data of EIA procedure, called Baseline Geodatabase (BGDB), is designed. The BGDB includes feature datasets (projects, climatology and air quality, hydrology, pedology, general location map of the study area, geology, noise, biology, and socio-economic-cultural data) and tables (fauna). For example, the BGDB for Sabalan geothermal project area, Iran as a case study is developed. This paper provides a practical tool to facilitate the EIA process, environmentally sustainable management, and to support decision-making for environmental specialists and managers in the present and future of the proposed project area.