The shattering habit of rice affects harvest efficiency and also is one of the important traits. The evaluation way of the habit has been demanded, and the mean value and variance have previously been used for this purpose. In this study, the shattering habit was evaluated by the strength distribution on the grain detachment force because it immediately reflected the habit. Experiments ware carried out for 49 days during the harvest season, and the grain detachment force was measured due to parallel pull (parallel foece) and perpendicular pull (perpendicular force). As a result, the mean value of the parallel force slightly increased through the experiments, and the shape of the strength distribution varied. Next, the Weilbull distribution was applied in order to evaluate the strength distribution. The Weilbull distribution fairly conformedd to the strength distribution on the grain detachment force, which was consisted of 945 samples. Also, the amouont of samples to determine the Weilbull distribution was discussed. The result showed that the Weilbull distribution could be accurately determined using about one hundred samples.