The study aimed to investigate the effect of an interdisciplinary STEM course on children's attitudes of learning and engineering design skills. A total of 449 elementary school children in China were recruited from three different grade levels (i.e., second, fourth and sixth grade) to participate in this study. All participants attended a weekly interdisciplinary STEM course for two consecutive semesters. In order to understand how an integrative STEM curriculum interplayed with participants' attitudes of learning and engineering design skills, we conducted two measures to assess the results: the attitude measure (i.e., the attitude of learning questionnaire) and the outcome measure (i.e., the STEM knowledge assessment and the scenario design assessment). Meanwhile, pre-and-posttests were implemented for each measure when comparing the differences in learning. Our findings revealed that an interdisciplinary STEM course led to positive changes in children's attitudes of learning, yet its effect on different grade levels could vary depending on children's cognitive development. In addition, interdisciplinary learning experiences significantly improved second and fourth graders' engineering design skills, particularly in the scenario design assessment. Lastly, we summarized the relationship between one's attitudes of learning and engineering design skills along with its educational implications.