The kinetics of martensite decomposition in TC21 alloy was investigated at isothermal conditions in the temperature range 500-850 A degrees C. The dilatometry technique was utilized to trace the transformation process for different aging temperatures. Within the framework of the Avrami theory, the analysis of the experimental data was made by means of the Johnson-Mehl-Avrami (JMA) equation. A very good correspondence between the calculated and the experimental results was found. The JMA kinetic parameters obtained from different aging temperatures implied different mechanisms of the transformation. The alpha aEuro(3) phase transforms to alpha A + alpha aEuro(3) (rich) at 500 A degrees C and the transformation is incomplete. Further increase of the temperature to 800 and 850 A degrees C results in directly transformation alpha aEuro(3)A a dagger'alpha A + beta. The mechanism of the transformation alters during the course of the transformation for 550, 600, 650, 700, and 750 A degrees C. Moreover, The TTT diagram was constructed for the martensite decomposition in TC21 alloy based on the dilatometry analysis and JMA theory, respectively. Good agreement between experimental and calculated TTT diagram is observed.