The thermal nitric oxide formation in a one-dimensional tubular flow reactor has been investigated. A mixture of oxygen and nitrogen was continuously supplied into an electrically heated reactor of wall temperature within the range 1653-1798K. Five different reactor diameters made of two materials were used. The measured rate of nitric oxide formation was much higher than the calculated by the Zeldovich thermal mechanism with the available rate constants. During the experiments no fuel was supplied into the reactor, thus the concept of the Fenimore prompt NO could not be used to clear up these discrepancies. Also the nitrous oxide mechanism with the recommended rate constants could not explain the results. The observed rate of nitric oxide formation has been described by a sequence of five reactions formed of the Zeldovich mechanism and the proposed by Malte and Pratt N2O mechanism: N-2 + O reversible arrow NO + N, (1) N + O-2 reversible arrow NO + O, (2) N-2 + O + M reversible arrow N2O + M, (6) N2O + O reversible arrow NO + NO, (7) N2O + O reversible arrow N-2 + O-2, (8) For which the rate constants of reactions (1) and (7) with the assumption of O radicals in equilibrium were tested. Because these reactions proceed independently of the combustion process it is proposed to call them as an extended thermal mechanism. The rate of nitric oxide formation through the extended thermal mechanism is much faster than by the Zeldovich thermal mechanism (reactions 1 and 2) within the flame temperature range. The role of nitrous oxide for the thermal NO build up is crucial in temperatures below 1800K, while of the Zeldovich mechanism (reactions 1 and 2) above 2000K.