Spherically expanding flames of natural gas-air mixtures have been employed to measure the laminar flame speeds, at the equivalence ratios from 0.6 to 1.4. initial pressures of 0.05, 0.1 and 0.15 MPa, and preheat temperatures from 300 to 400 K. Following, Markstein theory, one then obtains the corresponding unstretched laminar burning velocity after omitting the effect of stretch imposed at the flame front. Over the ranges studied. the burning velocities are fit by a functional form u(1) = u(10)(T-u/T-u0)(alphaT)(P-u/P-u0)(beta1)', and the dependencies of alpha(T) and beta(P) upon the equivalence ratio of mixture are also given. The effects of dilute gas on burning velocities have been studied at the equivalence ratios from 0.7 to 1.2. and the explicit formula of laminar burning velocities for dilute mixtures is achieved. (C) 2004 Elsevier Ltd. All rights reserved.