The purpose of this study is to utility the Theory of Planned Behavior (TPB) (Ajzen, 1985, 1988, 1991) to identify the determinants of regular physical activity/exercise behavior of youth in Hong Kong (HK). The questionnaire consisted of three sections: 1. questionnaire which was designed to elicit response on the five main constructs: a) attitude toward exercise, b) subjective norm toward exercise, c) perceived behavioral control of exercise, d) intention to exercise and e) exercise behavior; 2. physical activity/exercise participation pattern; and 3. demographic characteristics: gender and age. The total number of participants was 1946 [868 were males (45.5%) and 1040 were females (53.4%)]. They were divided into three age groups (11-14 years old=844, 15-18 years old =1002, 19-21 year olds=77). 480 students (24.9%) participated in physical activities regularly. The top five reasons for participation were:1. maintain health; 2. release stress; 3. improve mental health; 4. improve physical outlook; and 5. make/meet friends. 90 students (4.6%) never participated in physical activity/exercise within the past month and the reasons for not doing exercise were:1. do not have time; 2. feel tire; 3. lazy; 4. need someone to be with him/herself; 5. feel sick. 2 X 3 ANOVAs revealed that males participated in physical activity/exercise significantly more than females (F=5.4, p=.021) in the past month. There was age group difference in the physical activity/exercise participation rate (F= 29.3, p=.000). The oldest students did less physical activity/exercise than the other two groups, and the youngest students participated in physical activity/exercise the most. For the intention to do exercise in the coming month, more males agreed to do exercise than females (F=4.66, p=0.031) and the youngest students agreed to do exercise in the coming month than the oldest one. (F= 30.6, p= .000). The most popular physical activities were athletics, badminton and basketball. Regression analysis results revealed the physical activity/exercise behavior equation as follows: -1.245 + .312 (past experience) + .233 (perceive behavior control) + .106 (intention to exercise) + .118 (perceive behavior belief) -.059 (subjective norm) + .037 (attitude toward exercise). These variables could explain 46.7% variance in exercise behavior (Adjusted R square = .467) and was significant (F = 237.3, p =.000). Past experience, perceive exercise behavior control and intention to exercise were the major and significant contributors to exercise behavior. Physical educators should create positive exercise experiences for youth in order to encourage them to participate in physical activity/exercise.