The activities of the key enzymes involved in the intracellular poly-3-hydroxybutyrate (PHB) synthesis of Methylosinus trichosporium IMV3011 were studied under various cultivation conditions. The enzymes were methane monooxygenase (MMO),β-ketothiolase, acetoacetyl-CoA reductase, PHB synthetase, and PHB depolymerase. Each enzyme had a unique catalytic mechanism. MMO activity decreased continuously with PHB production, but PHB at a high concentration was beneficial for maintaining MMO activity because more NADH was released by PHB depolymerization. The important reaction for entering the PHB cycle was catalyzed byβ-ketothiolase. The monomer of β-hydroxybutyrate was synthesized by the catalysis of β-ketothiolase and acetoacetyl-CoA reductase. PHB synthetase played an important role in the PHB synthesis routes. The activity of PHB synthetase increased with PHB production. The changes of en-zyme activities involved in PHB synthetase and PHB depolymerase occurred together, which indicated that the polymerization and depoly-merization of intracellular PHB occurred simultaneously. The molecular weight of PHB was determined mainly by the combined actions of PHB synthetase and PHB depolymerase. Some important intermediates in the tricarboxylic acid cycle were helpful for PHB production be-cause they increased related enzyme activities in the PHB cycle.