An investigation was made of some factors affecting the weight fractions of wood pulp separated with H.S. type fiber classifier.
1) The residue on the sieve decreases rapidly as screening starts. The rate of decrease which is related with the ratio of mean fiber length of the sample to mesh size, falls off gradually in process of screening, but generally does not reach to zero.
2) The considerable effect due to water pressure of the sprinkler and sample weight was found. Those operating conditions should be fixed, e.g. water pressure 1 kg/cm2, sample weight 5 g and sieving time 5 to 10 min. were suitable.
3) The effect of pulp defibrating prior to the test was relatively so small, that slush pulp could be tested without disintegration.
4) The approximate linear relations were found among a fraction and others which classified with the different meshes, therefore the measure of mean fiber length of the sample might given briefly with the result of one determination for the residue, when the classifier with a single sieve is applied.The schematic expression of this relationships was represented.

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