The biodegradable plastic poly-beta -hydroxybutyrate (PHB) was synthesized by a unicellular cyanobacterium, Synechocystis sp. PCC6803. The cell growth and PHB accumulation were inspected comprehensively. Nitrogen starvation was favorable for cyanobacterial PHB accumulation, while the final biomass of nitrogen-starved culture decreased to one-third of that in nitrogen-sufficient condition. A two-step culture, a balanced mixotrophic culture followed by a nitrogen-starved culture, was developed to improve the cyanobacterial PHB production efficiency. To adapt the large-scale outdoor culture that has the natural light/dark cycle, modification to the culture method was put forward based on the studies on the heterotrophic culture.