In the study, the effects of STEM activities applied to primary school fourth grade students on students' attitudes towards STEM, 21st century skills and mathematics achievement were analyzed. Sequential exploratory design was used in the research, which was carried out as a mixed method. In the quantitative part of the research, time series design, one of the semi-experimental designs, was used. The study group consists of 34 primary school fourth grade students. Quantitative data were collected with the STEM Attitude Scale, the 21st Century Creativity and Renewal Skills Scale, the Scratch Achievement Test and the Mathematics Achievement Test. On the other hand, qualitative data were collected with a Semi-Structured Interview Form. Descriptive statistics, ANOVA for Repeated Measures, and dependent groups t-test statistics were used in the analysis of quantitative data. Qualitative data were analyzed using descriptive and content analysis techniques and presented in tables. As a result of the research, it was determined that STEM education has a significant effect on the attitudes of primary school fourth grade students towards STEM and their 21st century skills. It has been figured out that the effect of STEM education on increasing the mathematics achievement of primary school fourth grade students is not significant. Students reported that STEM education activities were fun, useful and instructive. With STEM implementations, students' attitudes towards science, technology, engineering and mathematics have changed positively; They stated that 21st century skills such as communication, cooperation, creativity and critical thinking have developed.