Abstract: To investigate the relationship between personality trait and defensive mechanism among college students of science and engineering. Methods: Using EPQ-RSC (Eysenck's Personality Trait Questionnaire - Revised, Short Scale for Chinese), DSQ (Defensive Style Questionnaire) , 207 students were selected by randomized block sampling and tested. Results: The students with psychoticism personality trait trend is apt to use much more unmature and less mature defensive mechanism. Ones with neurotism have much unmature, and intermediate defensive mechanism. Ones with extroversion are used to adopt in mature and some intermediate defensive mechanism at the same time. There is some difference between male and female students with psychoticism personality trait. This study suggests that personality trait type often accords to certain defensive mechanism. There is a complex interrelation between personality trait and defensive mechanism in college students of science and engineering. At the same time, there is some difference between two genders and among four grades and professions in defensive mechanism and Personality trait.