The aim of the study was to investigate the effect of a physical fitness intervention program within a physical education class on cardiovascular endurance of Malaysian secondary school boys and girls. A quasi experimental design was adopted for the study. Two schools in a district were randomly selected. In each school, two classes were randomly assigned intact to the experimental group of boys and girls (n = 45; n = 48) respectively and the other was the control group of boys and girls (n = 42; n = 38) respectively. Data were collected on cardiovascular endurance using Cooper's Twelve Minute Run. The experimental and the control groups underwent regular physical education classes twice a week for ten weeks. Apart from the regular physical education classes, the experimental group of boys and girls underwent the treatment of four exercises in a form of a circuit immediately after the warm-up session. After ten weeks, posttest data were collected. ANCOVA indicated that there was a main effect in cardiovascular endurance F(1, 84) = 18.17, p < 0.05; Cohen d = 0.17 between the boys and also a main effect F (1,83) = 44.69, p < 0.05; Cohen d = 0.35 between the girls. The results indicate that a ten week physical fitness program implemented within a physical education class was effective in enhancing cardiovascular endurance among the Malaysian secondary school boys and girls.