It has been occasionally explained that the angle of shear resistance depends on the soil-particle size. However, the qualitative relationship between the friction coefficient and the particle size is still controversial. To elucidate the effect due to particle size, the samples with some kinds of particle sizes should be prepared by sieving. It is not ideal to get samples by crushing, because the shape of the grain becomes quite different from the original one. Also, the shear experiments should be done under the low stress level, where the particles are not crushed. To obtain the relationship between the particle size of soil and the friction coefficient, the direct shear tests were performed under the low constant normal stress less than ca. 230 gf/cm^2 with some uniform particles of three sizes. As the result, it is found that the friction coefficient increases monotonously with the increase of the particle size. This concludes that the friction coefficient (tan φ) can be expressed by only two factors; mean diameter (d_<50>) and relative density (Dr).