For the establishment of a more general and comprehensive activated sludge stoichiometry, which can be applied to the treatment of complex organic wastewater, experimental studies of activated sludge process were carried out. Changes in the elemental composition of activated sludge with the change of sludge retention time (SRT) were investigated. Elemental composition of activated sludge treating synthetic organic wastewater composed of peptone and yeast extract was constant at C5H9NO3 over a wide range of process operating conditions. A definite elemental composition of activated sludge including phosphorus cannot be successfully obtained because of the variations of phosphorus content. Average chemical compositions of activated sludge which was cultured below SRT of 3 d and above SRT of 5 d were C60H114N13O40P2 and C60H113N13O37P, respectively. Yield coefficient (Y) and decay constant (b) were experimentally determined to be 1.22 g-MLSS/ g-TOC and 0.1261/d, respectively. Using the experimentally determined kinetic constants, stoichiometric equation for the treatment of complex organic synthetic wastewater can be successfully developed.