The kinetic details of segment formation in living radical polymerization (I-RP) were investigated by studying styrene (S)/methyl methacrylate (MMA) copolymerization via nitroxide-mediated controlled radical polymerization (NM-CRP) using kinetic Monte Carlo (KMC) simulations. It was demonstrated that the classic theory describing the distribution of instantaneous segment lengths developed for conventional free radical polymerization (FRP) does not always hold in LRP. Segment formation can be controlled by deactivation through coupling of growing radicals and nitroxide radicals instead of selective preference during propagation due to different reactivity ratios and composition of competing monomers if the transient lifetime is too short. Mathematical equations were proposed for determining in a facile way whether the segment growth is controlled by deactivation under given reaction conditions. The uniformity of segment growth was analyzed as well. It takes a surprisingly long time to allow the majority of the chains to be reactivated at least once in an S-rich reaction system, which leads to high polydispersity in monomer sequences formed on different chains. It was also demonstrated that there inevitably exists a distribution of segment lengths at it given monomer composition no matter how "ideal" the livingness is. This distribution is determined by the "intrinsic" kinetics of copolymerization and is not manipulated by the Features of I-RP.