Experiments on the transmission error caused by service time of polyurethane synchronous belts were carried out under initial tension when the center distance of the experimental apparatus was fixed. The experimental results were compared with the computed results on the fracture of the belt. It was confirmed that uneven wear of the belt flanks and root lines for one revolution of the belt due to running fatigue tests was caused by production errors in the belt as a single pitch error, a change in cross-sectional area of the tension members and a change in modulus of elasticity. As the experimental results agreed quite well with the computed ones, the transmission error due to belt running was increased by the uneven wear in the belt flanks and it was hardly affected by uneven wear in the root lines. In addition, it was found that the change in the transmission error due to the service time can be reduced when production errors of the belt are lessened.