Kinetics of polymerization of butadiene by ethyllithium in benzene have been studied by means of dihtometer． It has been shown that the polymerization rate iS first order in monomer concentration and 1／6 th order in ethyllithium concentration，when the con-centration of ethyllithium is in the range of 0．0045—0．0215 mole／liter． within 10-30℃，the activation energy of polymerization has been determined to be 19+0.5 kcal/mole．The rate of initiation has been determined by measuring the volume of ethane evolved after hydrolyzing the ethyllithium unconsumed in polymerization．The results show that when the conversion reaches about 10％，90％of ethyllithium is consumed in initiation Molecular weight distribution of the product as determined by fractional precipitation is shown to be very narrow． Addition of small quantity of tetrahydrofuran greatly increases both the polymerization rate and the initiation rate． Ethyl ether or triethylamine likewise increases the polymerization rate but less strongly．Anisole．diethyl sulfide or naphthalene almost show no effect on the polymerization rate．