The molecular weight of poly-hydroxybutyric acid (PHB) produced by Alcaligenes sp. K-912 is an important parameter characterizing the physical properties of the polymer. The effects of temperature and the levels of glucose, ammonium, phosphate and amino acids on the molecular weight of PHB were investigated. Molecular weight of PHB by temperature varied in the range of 380,000 to 550,000, 400,000 to 600,000 by glucose, 300,000 to 380,000 by phosphate, 400,000 to 1,000,000 by amino acids, respectively under the experimental conditions.