We introduce a simple model based on the Moran process with network dynamics.Using pair approximation,the cooperation frequencies at equilibrium states are deduced for general interactions.Three usual social dilemmas are discussed in the framework of our model.It is found that they all have a phase transition at the same value of cost-to-benefit ratio.For the prisoners dilemma game,notably it is exactly the simple rule reported in the literature[Nature 441 (2006) 502].In our model,the simple rule results from the parent-offspring link.Thus the basic mechanism for cooperation enhancement in network reciprocity is in line with the Hamilton rule of kin selection.Our simulations verify the analysis obtained from pair approximation.