Primary education is an essential stage and has an important impact on students' learning attitudes throughout the coming school years. The research explored the attitudes towards science, technology, engineering and mathematics among students through all grade levels in primary school. The Project-based Integrated STEM Program was proposed to study the changes of primary students' attitudes towards STEM. An assessment of S-STEM which consists of the STEM subscale and the 21st century skills subscale was utilized for both pre-test and post-test. The results showed that primary students exhibited little different attitudes on the S-STEM in the pre-test, regardless of gender and grade level. As evident from the comparison between the pre-test and post-test, the Project-based Integrated STEM Program had a positive effect on student attitudes towards STEM. It is encouraging if there are more STEM-related programs implemented at all educational stages covering the primary level.