This study is part of a five year National Science Foundation (NSF) funded project, Transforming Elementary Science Learning through LEGO™ Engineering-design (TESLED). In this study, we report on the successes and challenges of implementing an engineering-design based and LEGO™ oriented unit in an urban classroom setting, and we focus on the impact of the unit on students' content understanding of simple machines. The LEGO™ engineering-based simple machines module, which was developed for 5th graders by our research team, was implemented in an urban school in a large city in the Northeastern region of the US. Thirty three fifth grade students participated in the study, and they showed significant growth in content understanding. We measured students' content knowledge by using identical paper tests and semistructured interviews before and after instruction. Our paired t-test analysis results showed that students significantly improved their test and interview scores (t = -3.62, p < 0.001 for multiple-choice items and t = -9.06, p<0.000 for the open-ended items in the test, and t=-12.11, p<0.000 for the items in interviews). We also identified several alternative conceptions that are held by students on simple machines.