In this study, a new competency assessment system was developed for the blended learning course "Organic Chemistry Laboratory". The system utilized a mastery transcript evaluation model and incorporated the Structure of Observed Learning Outcomes (SOLO) taxonomy and Primary Trait Analysis (PTA) scale method. With this system, students can clearly understand the proficiency level that they need to reach both online and in class to achieve their desired course grades. The impact of the assessment method was analyzed by examining 20 sophomore students from a university. The findings highlighted that this new competency-based rubric offers several advantages, such as providing a more comprehensive assessment of students' abilities, encouraging the development of their enthusiasm for independent learning, and improving their concentration during experiments. All of these benefits contribute to developing their overall competency skills.