Infusing creative thinking competence through the design process of authentic projects requires not only changing the teaching methods and learning environment, but also adopting new assessment methods, such as portfolio assessment. The participants in this study were 128 high school pupils who have studied MECHATRONICS from 10th to 12th grades (16-18 years old). By the end of 12th grade, the pupils had created 57 authentic projects. The intervention program had two parts: first, the pupils documented their project according to a creative design process that had been introduced to them. Second, the projects were assessed according to a creative thinking scale. This scale was designed to assist pupils in documenting the design process. It could be used as a guideline for teachers and pupils during the course of the project. The research examined pupils' performance during project-based learning. The research tools included: observations of class activities, portfolio assessment, and external matriculation assessment. The findings show first that pupils learned to document their design process. Second, pupils' projects demonstrated various levels of creative thinking skill. Evidences for high-level documentation of the projects were found in pupils' portfolios. On the other hand, there is much to be learned about documenting teamwork and pupils' reflection. This research could assist researchers and teachers who are interested in assessing engineering education outcomes.