Objective Kt/V has been used to assess adequacy of peritoneal dialysis. However, Kt/V urea only reflects the clearance of solute without considering the effects of dietary protein intake (DPI). The objective of this study is to evaluate the value of Kt/V based on nitrogen balance and to study whether it is the best adequacy index. Methods On the premise that nitrogen balance is obtained, we calculate the minimal adequate dialysate volume and dialysis index (DI), Kt/V and solute removal index (SRI). We compared the values of DI、Kt/V and SRI in evaluating the adequacy of peritoneal dialysis. Results Kt/V changed with DPI and body weight in CAPD patient. We can not define adequate dialysis with a single value while DPI and body weight are different. For CAPD, SRI and Kt/V are numerically equal. However, dialysis index (DI) can reflect the minimal adequate dialysate volume when DPI and body weight changed and thus reflects the adequacy of dialysis.Conclusions DI reflects the balance between DPI and solute clearance. It is more objective to evaluate dialysis adequacy than Kt/V and SRI in clinical stable peritoneal dialysis patients.