To develop a method to estimate crop size of oak trees (Quercus crispula) with least labor and as quickly as possible, the method proposed by Koenig et al. (1994) was tested in temperate forests of Japan. In autumn 2007, thirteen oak trees with various degree of fruiting were selected. First, following Mizui (1991), the number of acorns at 50-cm ends of six branches at the upper position of a crown was averaged for each of selected trees (Mizui’s index), as the de facto standard of index for crop size of a tree. Second, following Koenig et al. (1994), the number of acorns was visually counted within 30 seconds using a binocular (Koenig’s index) by eight observers. Koenig’s index was measured with nine replications per an observer. It is shown that Mizui’s index (y) was well regressed by Koenig’s index (x) using a power function (y =ax^b); parameter a were estimated as 1.4∼1.8 and parameter b as 0.32∼0.37, with coefficients of determination of 0.44∼0.56 after adjustments for the degrees of freedom. Although variation among the observers and among real fields of view for binoculars were mostly insignificant, one of the observers significantly underestimate Mizui’s index compared with other observers, and another observer also tended to underestimate Mizui’s index when using binoculars of narrower field of view. For the application of Koenig’s method, 6 replication of observation per a tree will be enough, although cautions will be needed to calibrate counting ability between observers and to use the same binocular, if possible, for every observer.