Computational thinking has been gaining new impetus in the academic community and in K-12 level education. Scratch is a visual programming environment that can be utilized to teach and learn introductory computing concepts. There are some studies investigating the effectiveness of Scratch for K-12 level education. However, studies that have been conducted at the collegiate level, especially in the context of preservice computing teacher education, are very rare. The present study aimed to investigate the effect of Scratch-based instruction on preservice teachers' understanding of basic programming concepts and their attitudes toward programming. In the present study, a mixed method design was utilized. In the first phase of the study, the data were collected using an achievement test, a practice test, and a computer programming attitude scale. In the second phase of the study, data were collected through a semistructured interview. The results of the study indicated that preservice teachers in Scratch-based instruction had significantly better understanding of basic computing concepts. Qualitative data indicated that Scratch-based instruction was useful in constructing a more meaningful learning environment for preservice teachers. The results of this study have implications for researchers and preservice computing teacher educators when designing an introductory computing course.