A method of calibration of a particle characterization apparatus, and a particle characterization apparatus, in which particles suspended in a liquid are passed through an orifice one by one for characterization of the particles, for instance by Coulter counting. The calibration does not require utilization of special calibration particles or liquids. A priori knowledge of the shape of a typical size distribution of a blood sample is utilized to adjust the apparatus based on an initial relatively short counting period of the sample in question. The initially determined size distribution is compared to the corresponding known typical size distribution and the apparatus is subsequently adjusted to counteract possible differences. Upon adjustment of the apparatus, the remaining part of the sample is passed through the orifice for determination of the actual particle size distribution of the remaining sample.