This study aimed to assess the physical activity levels of pediatric patients with acute leukemia under-going chemotherapy. Thirty-eight pediatric patients and matched controls, aged 3–12 years old, were measured for weight, height, and other anthropometric parameters. Physical activity was assessed using actical accelerometer and activity log book. Patients recorded significantly lower mean total activity counts (26.2 ± 30.2 cpm vs. 192.2 ± 68.8 cpm; p < 0.01) and spent more time in sedentary activities (1301 ± 121 min vs. 1020 ± 101 min; p < 0.001) compared to controls. They also achieved fewer 1–5-min bouts of moderate–vigorous physical activity (MVPA) compared to controls (1.50 ± 5.95 vs. 37.38 ± 40.36; p < 0.001). In conclusion, patients had lower physical activity level and intensity; and simple exercise intervention programs may be needed to minimize the detrimental effects of prolonged sedentary behaviors.