The polymerization of actin has been studied with applying a new method of microcalorimetric measurement in vitro. The thermodynamic parameters of actin poly merization at 310.15 K is reported. △H_m= 49.26 kJ·mol~(-1)， △G=-25.62 kJ·mol~(-1) and △S=241.54 J·K~(-1)×mol~(-1). Thermogram showed that the thermokinetic process of actin polymerization had two different thermal effects where in the first exothermic step is con sidered to be the total results of several reations: metal ion (Mg~(2+), K~+) binding to actin; the change of conformation of actin and hydrolysis of ATP in actin; and the second endothermic step is assigned to the results of association and length adjustment (annealing process). Cisplatin, at lower concentration, affects the polymerization of G-actin reflected in the decrease of ΔH_m; and at higher concentration it induces the crosslinking and depolymerization of F-actin in the equilibrium system of G/G. The experimental results manifest that when the concentration of cisplatin is increased to a certain value, the sign of ΔH_m will be changed from positive to negative i.e. the magnitude and sign of ΔH_m of actin polymerization is dependent on the concentration of cisplatin. This is possibly related to the pharmacologic or toxicological action of cisplatin.