This study focuses upon the effectiveness of project-based learning on primary school pupils regarding their content knowledge and attitudes towards self-efficacy, task value, group work, teaching methods applied and peers from diverse ethnic backgrounds. A cross-curricular project was implemented within the curriculum area of environmental studies under the title of "sea animals". The methodology applied in this study was the quasi-experimental research design. The findings of the present study support the view that pupils can gain benefits through project-based learning in obtaining content knowledge and group work skills and that they became less favourable to traditional teaching versus experiential learning. Motivation (self-efficacy and task value in terms of environmental studies) and developing positive attitudes towards peers from a different ethnic background were changed in moderate levels after the project. (Contains 2 tables.)