In the study, self-assessment scale of design process was developed for secondary school students. In addition, the relationship between perception for problem solving skills and decision-making attitudes, which are considered important in the design process, were examined. This study was based on the relational screening model and purposive sampling method was preferred. The research was carried out in two stages, in the fall and spring term of the 2018-2019 academic year. In the first stage, the scale was developed and in the second stage, the relationship between the scale and other variables was examined. The first stage of the study was conducted with 530 students from 7th and 8th grades, whereas 447 students participated in the second stage. In this study, Self-Assessment Scale of Design Process, Perception Scale for Problem Solving Skills and Decision-Making Scale in Adolescents were used. Descriptive statistics, Exploratory and Confirmatory Factor Analysis, Pearson Correlation Coefficient, and Multiple Regression analyzes were used in data analysis. As a result of the research, a valid and reliable measurement tool that can be used for self- assessment of design process has been developed. Moreover, it was concluded that problem solving perception and cautious-selective decision-making style have significant contribution on the self-assessment of the design process.