The influence of crystal orientation on measured size is discussed for comparison between crystal size distribution (CSD) obtained by simulation and the ones by experimental measurement. In the simulation in which the CSD is calculated, the size should be defined as a characteristic dimension. On the other hand, the measured size to be compared with the calculation result does not always represent the same definition of crystal size. In most cases, the measured value is affected by crystal orientation, crystal shape and the measurement method itself. Considering this situation, the gap between the characteristic size and measured size were estimated by numerical calculation in the case of sieve diameter and Feret's diameter, which are widely used in crystallization analysis. The estimation was proved to be reasonable by testing the experimental data. It is implied to be important to evaluate the effect of crystal orientation on measured size when comparing simulated CSD and actual data.