Science, technology, engineering, and mathematics (STEM) are the key to Egypt's global economic competitiveness. STEM education prepares skilled workforce, creates critical thinkers, increases science literacy, and enables the next generation of innovators. The purpose of the present study is to examine the effectiveness of a hands-on summer STEM program in developing middle school students' design thinking and conceptual understanding. To accomplish this purpose, a list of bases that should be taken into account while designing the summer STEM program was prepared. An instrument for design thinking and a test for conceptual understanding were developed. These instruments were administered to 28 students- who completed 8th grade- before and after participating in the summer STEM program. The findings showed that students' design thinking developed after the STEM program. On the other hand, the program increased the students' conceptual understanding. These findings indicate that the focus should be on the implementation of informal hands-on experiences to increase interest in STEM disciplines and improve the students' design thinking and conceptual understanding.