Abstract: Objective To investigate the role of on-line blood volume monitoring on prevention of dialysis-Symptomatic hypotension and dialysis-refractoy hypertension during hemodialysis. Methods 31 cases of hemodialysis patients were divided into three groups according to their blood pressure: normal blood pressure (NBP) group (n = 9), symptomatic hypotension (SH) group (n = 12), refractory hypertension (RH) group (n = 10), and observed the relative blood volume(RBV), blood pressure (BP), ultrafiltration volume (UV), mean arterial pressure (MAP)by on-line blood volume monitoring. The Research carried out in three stages, stage 1 and stage 2 for the observation period, the third stage for the intervention period, then comparatively analysis of the changes of the indicators. Results The RBV changes in NBP, SH, RH groups were significant different (P <0.05); by increasing dry weight or changing ultrafiltration mode of SH group, the incidence of SH was significantly decreased (P<0.05), by enhancing ultrafiltration of RH group,the MAP before or aftre hemodialysis was significantly decreased (P <0.05). Conclusions The RBV changes are different in HD patients with different blood pressure, on-line blood volume monitoring could provide important information for the set of dry weight,do good to formulated individual dialysis programmes , and effectively reduce the occurrence of dialysis-SymPtomatic hyPotension, low down the MAP of dialysis-refractoy hypertension patients,then improve the dialysis security. on-line blood volume monitoring was worthy of clinical promotion.