To assess the effectiveness of the cadaver-based educational seminar for trauma surgery (C-BEST) for residents using a 10-point self-assessment of confidence levels (SACL) survey. We collected data, including SACL for 21 surgical skills and an evaluation of the contents before, immediately after, and half a year after the seminar, from 42 seminars conducted between January 2013 and March 2019. On comparing SACL results from evaluations at the three time points using statistical analysis, a p value of <0.05 was obtained. We enrolled 412 participants; of the 52 residents, 47 respondents were included in the study. Improvements in all skills were observed on comparing SACL before and immediately after the seminar (2.2 ± 2.6 vs. 5.3 ± 2.5; p < 0.001), but a decrease was observed between immediately after and half a year after the seminar (5.3 ± 2.5 vs. 4.9± 2.7; p < 0.01). Upon examining the results according to each skill, SACL did not decrease between immediately after and half a year after the seminar (p > 0.05). The most performed procedure was left anterior thoracotomy and aortic clamp (n = 13), and the number of residents majoring in surgery increased from 27 to 32 half a year after the seminar. C-BEST boosts more self-confidence of the participating residents immediately after the seminar. Although this effect is not maintained half a year after the seminar, many participants practiced their skills after the seminar. Therefore, C-BEST is useful for residents with little surgical experience; this may also inspire the residents to major in surgery.