Physical Education (PE) can foster regular physical activity (PA) in children. However, children engage in insufficient moderate to vigorous PA (MVPA) during PE. This study objectively investigated MVPA of children during a single, compared with double PE-period. In 294 children (7.1 ± 0.7 years) PA was objectively assessed. PE periods were determined and PA was individually calculated. Children spent 8.5 ± 7.3 minutes of each 45 minutes PE lesson in MVPA. Boys were significantly more active than girls (p = 0.01). All children participated in 135 minutes PE/week, 32.7% were scheduled one double and one single PE-period. Children, with a double PE-period and one single lesson engaged in significantly less MVPA than children, who had three single periods of PE (6.7 ± 6.9 minutes/45 minutes vs. 9.4 ± 7.4 minutes/45 minutes, respectively; p = 0.01) In conclusion, single periods of PE seem to be more effective in getting primary school children to engage in more MVPA than one double period per week.