This paper deals with a combustion stability of diffusion flame. A simplified linear differential equation of second order, which involves the parameters estimated from the states of steady combustion, has been suggested to discuss theoretically the nature of oscillatory combusion about a two-dimensional combustion chamber. Its validity has been testified by comparing calculated results with experimental ones. Results obtained have indicated that factors markedly affecting the low-frequency oscillatory combustion are primary volumetric air-fuel ratio, duct length of combustion side, shape of burner and inlet throttles. But outlet throttle gives little effect on the stability of combustion.