In vitro experiments to investigate the hematocrit effect on human blood flow in microcirculation were carried out using a micro-PIV technique. The micro-PIV system consisted of a 2 head Nd:YAG laser as a illumination light, a cooled CCD camera, a delay generator and a personal computer for control and data processing. Human blood with a hematocrit of 20, 30 and 40% was supplied into a microtube of 100 microM in diameter using a syringe pump. Fluorescent particles of 1.0 microM in mean diameter were seeded in the blood flow as tracers to measure instantaneous velocity fields by applying a cross-correlation PIV algorithm. The mean velocity field information was obtained by ensemble averaging the instantaneous velocity field data obtained. The hemorheological characteristics related with the blood flow in the microtube were also evaluated as functions of flow rate and hematocrit using the PIV data. The blood flow has a cell-free layer near the tube wall and this layer's thickness is increased with the increasing flow speed due to the radial migration. As the hematocrit increases, the velocity profile starts acquiring non-Newtonian features under low flow rate conditions. The hemorheolgical characteristics were found to influence largely on the viscosity and shear rate of blood flow.