Abstract: In order to obtain the nodal hydrograph suitable for the unsteady flow simulation of urban sanitary sewer networks, the study on the dynamic characteristics of sewage flow series is conducted using fractal theory so as to judge whether there are fractal traits. Firstly, the delayed-time method is used to reconstruct the phase space of sewage flow series and calculate the correlative dimension. Then case studies are carried out based on the daily flow records derived from two wastewater treatment plants (WWTPs) in Beijing. In addition, the correlative dimensions are calculated respectively according to the delayed-time τ =Δt and τ = 4Δt. The results indicate that (1) the fractal traits exist in sanitary flow series; (2) the distinct linear section occurs within ln C(r, m) - ln r curv.es and the correlative dimension is 0.49 for WWTP A and 0.53 for WWTP B when τ= Δt;(3) the distinct linear section also occurs in ln C(r, m) - ln r curves whenτ = 4 Δt and the correlative dimension is 0.75 for WWTP A and 0.44 for WWTP B;(4) the correlative dimensions of the flow series quantitatively show that the flow source of WWTP is simplex.