It is believed that whether the instantaneous objective function curves of plug-flow-reactor (PFR) andcontinuous-stirred-tank-reactor (CSTR) overlap or not, they have a consistent changing trend for complex reactions(steady state, isothermal and constant volume). As a result of the relation of the objective functions (selectivity oryield) to the instantaneous objective functions (instantaneous selectivity or instantaneous reaction rate), the optimalreactor network configuration can be determined according to the changing trend of the instantaneous objectivefunction curves. Further, a recent partition strategy for the reactor network synthesis based on the instantaneousobjective function characteristic curves is proposed by extending the attainable region partition strategy from theconcentration space to the instantaneous objective function-unreacted fraction of key reactant space. In this paper,the instantaneous objective function is closed to be the instantaneous selectivity and several samples are examinedto illustrate the proposed method. The comparison with the previous work indicates it is a very convenient andpractical systematic tool of the reactor network synthesis and seems also promising for overcoming the dimensionlimit of the attainable region partition strategy in the concentration space.