In this paper, we present a conductivity method for measuring the RBC (red blood cell) aggregation-sedimentation phenomenon and the theoretical analysis of this phenomenon. A theory describing the RBC aggregation-sedimentation phenomenon with the conductivity method is proposed and the formula of the l-t function which depicts the RBC aggregation-sedimentation process is given. We put forward the aggregation index and sedimentation factor describing the RBC aggregation-sedimentation phenomenon for the first time.