The accuracy of a uniaxial accelerometer for assessing physical activity in preschool-aged children was assessed by using an indirect calorimeter which provides the physical activity ratio (PAR) for free living activities. Subjects were 5- to 6-yr-old Japanese girls and boys (n=24, 6.1±0.3 years). PAR was assessed for nine activities (lying down, watching a video while sitting and standing, line drawing for coloring-in, playing with blocks, walking, stair climbing, ball toss, and running) using the Douglas bag method. “Exercise intensities” were recorded with the uniaxial accelerometer (Lifecorder EX ; Suzuken Co. Ltd, Nagoya, Japan). PARs were also predicted by using the equations presented by Higuchi et al. (2003) and Kumahara et al. (2004). Significant correlation was observed between “exercise intensities” as measured by the uniaxial accelerometer and PAR for all activities (r=0.827). Predicted PAR values for walking and running were overestimated according to the equations. On the other hand, PAR values for stair climbing and ball toss were underestimated. These findings indicate that although the uniaxial accelerometer may help in evaluating daily physical activity in preschool-aged children, its use as a proxy measure of PAR based on the above equations may be limited.