This study examines reaction orders for concentrations of catalyst (tin octoate) and initiator (n-dodecanol) in the rate equation of PLA ring-opening polymerization. At first, the rate equation was proposed with their orders taken into account. Then, polymerization experiments were carried out on a flask scale to measure the time dependences of temperature and conversion ratio. The orders and the reaction rate constant were decided using the experimental results and mean square method. Reaction rate was nearly proportional to concentrations of both tin octoate and n-dodecanol. The Delta E-x and k(0) were 95.4 kJ/mol and 1.35 x 10(5) m(3)/(mol center dot ppm center dot s), respectively.