Optical coherence tomography (OCT) has the potential to be a noninvasive method for hematocrit (HCT) measurement. This study shows the effect of erythrocyte aggregation at the level seen in healthy humans and pathological states on HCT measurement based on monitoring the slope changes in OCT depth reflectivity profile using spectral-domain optical coherence tomography (SDOCT). Our measurement indicates that the HCT estimated by SDOCT depends on the erythrocyte aggregation state and the flow rate. Measured HCT in blood samples with 0.6% and 2% dextran 500 is underestimated by 4.5%-10.5% and 17.1%-19.5% for HCT from 35%-55% at a flow rate of 4.7 mm/s. Underestimation is smaller at a high flow rate as compared to a low flow rate, indicating erythrocyte aggregation is an important factor that will affect accurate HCT measurement using SDOCT.