Cylindrospermopsis raciborskii and Staurastrum spp. are the common dominant species in (sub) tropical reservoirs. To understand the effect of iron on seasonal dynamics and growth C. raciborskii and Staurastrum spp., we investigated and analyzed the driving effect of iron on the biomass and the relative biomass of them, in a typical tropical reservoir. And we performed batch culture experiments with C. raciborskii N8 and Staurastrum sp. FACHB-1449 under three iron concentrations (0.029 mg/L, 0.29 mg/L, 0.689 mg/L) with two kinds of phosphorus source respectively, and we compared the difference of the specific growth rate, alkaline phosphatase activity and siderophore between the two species. The results showed that, C. raciborskii and Staurastrum spp. are the dominant genera in phytoplankton community, the highest proportion of the both was 82% in phytoplankton biomass; the different of the response to environmental conditions mainly reflected in the changes of biomass with iron concentration between the two genera, such as the significant linear regression was detected between C. raciborskii biomass and dissolved iron concentration, but not in Staurastrum spp. biomass. In culture experiments, the maximal specific growth rate of C. raciborskii N8 was 0.098 d^-1±0.01 d^-1 at iron concentration of 0.689 mg/L with inorganic phosphorus and it significantly decreased with the decrease of iron concentration in both phosphorus source condition, concurrently, siderophore was detected in all of the treatments with C. raciborskii N8; comparatively, the specific growth rate of Staurastrum sp. FACHB-1449 was no significant difference among the six treatments, which was 0.079 d^-1±0.001 d^-1 in all of the treatments, and no siderophore was detected in all of the treatments, which mean the phosphorus source and iron concentration did not significantly effect on the specific growth rate. In addition, the alkaline phosphatase activities was significantly higher in the treatments with C. raciborskii N8 than in the treatments with Staurastrum sp. FACHB-1449 under organic phosphorus source, and the alkaline phosphatase activities in the treatments with C. raciborskii N8 significantly decreased with the decrease of iron concentration, but not in the treatments with Staurastrum sp. FACHB-1449. It was concluded that the supply of dissolved iron was critical to the seasonal dynamics of biomass and dominance of C. raciborskii, the growth of C. raciborskii is more vulnerable than Staurastrum spp. to iron concentration decrease, and the limiting effect of iron on C. raciborskii will be enhanced under inorganic phosphorus deficiency condition.