The formation and expansion of necklace structure in an Al-10Mg alloy have been studied. The alloy was produced by centrifugal casting, which was followed by 673K/24h solid solution treatment and hot uniaxial compression. The different effects of temperature and strain rate on expansion of necklace structure were especially clarified. In this study, bulging is the nucleation type of first layer in necklace structure, and three different types of subboundary were found in the deformation microstructure of Al-10Mg alloy. When temperature increased, microband and recovered subboundary were gradually eliminated, after which strain-induced subboundary nucleation and recrystallized grain coarsening became a main way of necklace expansion. When strain rate decreased, low angle grain boundary was accumulated. Three regions denoted by low misorientation region in interior, new grains in out layer and net structure of subboundary concentration region in middle were observed in non-recrystallized grains. The proportion of recrystallized grain area decreased when strain rate changed from 6.94×10_-1/s to 6.94×10_-2/s, then significantly increased when strain rate changed from 6.94×10_-2/s to 6.94×10_-3/s.