The circulating tumor cell test is used to evaluate the condition of breast cancer patients by counting the number of cancer cells in peripheral blood samples. Although microfluidic systems to detect or separate cells using the inertial migration effect may be applied to this test, the hydrodynamic forces acting on cancer cells in high hematocrit blood flow are incompletely understood. In the present study, we investigated the inertial migration of cancer cells in high hematocrit blood flow in microchannels. The maximum hematocrit used in this study was about 40%. By measuring the cell migration probability, we examined the effects of cell-cell interactions, cell deformability, and variations in cell size on the inertial migration of cancer cells in blood. The results clearly illustrate that cancer cells can migrate towards equilibrium positions up to a hematocrit level of 10%. We also performed simple scaling analysis to explain the differences in migration length between rigid particles and cancer cells as well as the effect of hematocrit on cancer cell migration. These results will be important for the design of microfluidic devices for separating cells from blood.