Background: Extracellular volume (ECV) assessment by bioimpedance analysis is a reliable technique for determining post dialysis target weight. Overhydration and dehydration are two points of dialysis patients’ fluid status and both affect blood pressure. We compared ECV and blood pressure between hemodialysis and peritoneal dialysis patients. Methods: We studied ECV of 74 (38 females and 36 males) normal subjects, 121 (63 females and 58 males) stable chronic hemodialysis and 84 (57 females and 27 males) stable chronic peritoneal dialysis patients. ECV as a percentage of body weight was designated ECV%. An ECV% over 28% in male patients and over 25% in females was defined as overhydration according to the 100th percentile of normal subjects. An ECV% below 21% in male patients and below 18% in females was defined as dehydration. Hypertension was defined as systolic blood pressure >140 mm Hg or diastolic blood pressure >90 mm Hg. Results: In male and female hypertension and normotension, the ECV% of peritoneal dialysis patients was significantly higher than that of hemodialysis patients (all p < 0.0001). The overhydration frequency of peritoneal dialysis patients was significantly higher than that of hemodialysis patients (p < 0.0001). The dehydration frequency of peritoneal dialysis patients was significantly less than that of hemodialysis patients (p < 0.01). The proportion of hypertension in peritoneal dialysis patients was higher than that in hemodialysis patients, but with no significant difference (p = 0.085). All male and female patients with overhydration had hypertension in either the hemodialysis or peritoneal dialysis group. Patients with dehydration usually had normotension, but some of them had hypertension. Conclusions: (1) Overhydration and hypertension are more common in peritoneal dialysis patients than in hemodialysis patients. (2) Dehydration is noticed in hemodialysis patients, but not in peritoneal dialysis patients.