Abstract: In recent years, genetic engineering of cyanobacterium has been developed to produce important chemicals, especially pharmaceuticals. The unicellular cyanobacterium Synechococcus sp.PCC7942 is an obligate photoautotrophic blue-green alga and a transformable strain that can be used as an effective host in genetic manipulation. There have been numerous studies on the expression of exogenous proteins in Synechococcus sp.PCC7942. In order to explore the possibility of protein pharmaceutical production with transgenic microalgae,Zhang et al cloned the human thymosin α1 gene that was expressed as a fusion protein with ubiquitin in Synechococcus sp.PCC7942. High cell-density culture of transgenic Synechococcus sp.PCC7942 with high expression level is very critical to its application, but few papers have been published on this aspect.In this paper, the transgenic Synechococcus sp.PCC7942 was batch-cultured in 1.5 L air-lift photobioreactors, and effects of CO2 concentrations on the growth, thymosin α1 expression and photosynthesis of the transgenic Synechococcus sp.PCC7942 were investigated. The specific growth rate of the alga cells during exponential growth phase was not significantly affected by CO2 concentrations; biomass and linear growth rate of transgenic Synechococcus sp.PCC7942 under normal CO2 and high CO2 concentrations were only slight difference.High CO2 level reduced NO3- assimilation and increased nitrate reductase activity, which indicated that reduction and assimilation of nitrate were uncoupled. Low CO2 level did not affect cell biochemical composition and thymosin α1 expression; however,the contents of soluble protein, chlorophyll a, carotenoids and phycocyarin of the transgenic alga cells grown under high CO2 level were significantly diminished, the thymosin α1 content was also markedly reduced at high CO2 level. On the basis of P-I curves, no difference was observed in photosynthetic efficiency; however, maximum photosynthetic rate was markedly improved by high CO2 level.