The calculation formulae of estimating the critical rate of temperature rise (dT/dt)_Tb for thermal explosion of exothermic decomposition reaction system under adiabatic,nearly adiabatic,apparent empiric-order autocatalytic and first-order autocatalytic conditions are derived from the relationship between the energy change of a reacting system and the extent of the reaction,and the sufficient and essential conditions from thermal decomposition to thermal explosion and the kinetic equation of non-isothermal reaction. The corresponding methods for estimating the values of (dT/dt)_Tb are presented.