This study was purposed to explore the effect of hyperthermia on sensitivity of multiple myeloma cells RPMI 8226 to adriamycin (ADM) and its mechanism. The working concentration of ADM against RPMI 8226 cells was defined by MTT assay. RPMI 8226 cells were divided into 4 groups: control group, hyperthermia (42°C) group, chemotherapy (ADM) group and combination group (42°C + ADM), the survival rate of RPMI 8226 cells in 4 groups was detected by trypan blue exclusion, the inhibitory effect of hyperthermia on proliferation of RPMI 8226 cells was detected by MTT assay, the cell cycle distribution, apoptosis rate of cells, intracellular ADM concentration and P-gp expression level were measured by flow cytometry. The 1/4 IC(50) of ADM was defined as the working concentration in the experiment. The results indicated that the hyperthermia promoted the entering the cells from in G(0)/G(1) phase into S and G(2)/M phases, the expression of P-gp protein on cells in hyperthermia and combination groups was down-regulated, the intracellular ADM concentration in combination group obviously increased. It is concluded that the hyperthermia combined with ADM obviously enhance the inhibitory effect on proliferation of RPMI 8226 cells. The hyperthermia increases the sensitivity of RPMI 8226 cells to chemotherapy through down-regulating the expression of P-gp protein on cells and increasing the intracellular ADM concentration.