Background: Improving students' decision making and judgment skills becomes an essential part of nursing education. The purpose of this Quasi-experimental study was to assess nursing students' clinical judgment in high-fidelity simulation based learning using observational measures. Method:Based on Tanner's clinical judgment model, a single group repeated-measures design was used with a purposive sampling at one nursing school in Macao. A total of 113 baccalaureate nursing students (49 in year two, 64 in year three) completed five simulation sessions within a 36 hour period. Two tutors used the Clinical Judgment Rubric to assess students' clinical judgment at the end of each simulation session. The inter-rater reliability for the sessions ranged from 0.833 to 0.910. Results:The scores for clinical judgment increased from the first simulation scenario to the last scenario. Compared with third-year students, second-year students had significantly higher scores of clinical judgment in scenarios two, three, four and five. The debriefing was essential to preparing students for improving their thinking and reasoning skills. Students perceived that simulation strengthened their theoretical knowledge and helped them to notice, interpret, and respond appropriately to contingencies and emergencies. Tutors' comments indicated that second-year students valued the newly learned knowledge and used it to analyze the simulated scenarios while third-year students presented less knowledge and skill preparation.Conclusions: High-fidelity simulation using a computer-controlled human patient simulator has potential to support the development of clinical judgment in nursing students. Tutors are challenged to enhance students' intrinsic motivation for learning, and encourage them to continue their efforts in learning. Further research is needed to explore whether senior students have better performance in clinical judgment than junior students.