The thermal theory of explosion has been applied to metal particle ignition. Critical conditions in the theta-eta plane for ignition are investigated. The analysis covers two kinetic laws of interaction between metals and gases, and uses the exact form of the Arrhenius expression for the rate of reaction law, as well as the Frank-Kamenetskii approximation. The relationship between the critical temperature and the gas temperature has been obtained from the definition d(2) theta/d eta(2) = 0 and d(3) theta/d eta(3) = 0, the other hand, the relationship between the critical parameters can be determined from the definition d psi(m)/d theta(m) = 0 or d psi/d eta(m) = 0.