STEM activities integrate various disciplines and are organized around domain-specific concepts and skills. What is achieved in terms of learning is related to how well the important STEM education characteristics are reflected in the implemented and integrated activities. This study aims to develop and evaluate a measurement tool for determining the characteristics of STEM activities by using a case study method. The STEM Activity Evaluation Form was developed based upon a literature review and experts' opinions. The observation-based form was applied to 21 different STEM activities, grouped in four categories: curiosity arousing activities, encouraging and upskilling activities, research and design projects, and one science fair. Students in science education partook in the activities in an eight-month period. The evaluation form has four categories: STEM learning environment, activitation of students, STEM content and practices, and connecting STEM. Application of the form revealed that the tool is useful to determine the appropriateness of activities with respect to the nature of STEM education.