Although plant density should affect soil carbon dioxide (CO2) efflux and carbon cycling in semi-arid regions, the effects of plant density on soil CO2 efflux are not well known. This study was performed to investigate the responses of soil CO2 efflux of two dominant shrubs (Caragana korshinkii and Salix psammophila) to plant density in the northern Loess Plateau of China. Two plant density treatments, low and high, were included for C. korshinkii (average 20,000 plants ha-1 and average 66,667 plants ha-1) and S. psammophila (average 9,583 plants ha-1 and average 31,250 plants ha-1). Soil CO2 efflux was measured every other day with an Ultra-light portable photosynthesis system (CI-340, CID Inc., USA) from July to October 2009. Soil CO2 efflux with high plant density was significantly larger than that with low plant density for both shrub species. Plant density did not change the temporal pattern of CO2 efflux during the study period. Our results indicated that root biomass and aboveground biomass were the significant biotic factors mediating the response of soil CO2 efflux to plant density for the two shrubs. Moreover, both soil water contents in the 0 - 6 cm soil layer and in deeper soil layer partly regulated the responses of soil CO2 efflux to the shrub density treatments in the semi-arid region.