Effects of nitrogen dilution on laminar burning velocities and Markstein lengths of propane-air mixtures were determined at the atmospheric pressure and room temperature based on the spherically expanding flames. The results show that, with the increase of the nitrogen dilution ratio, the burning velocity decreases and, for equivalence ratio less than 1.4, Markstein length increases with the increase of the dilution ratio, indicating that nitrogen addition decreases the preferential diffusion instability. The density ratio decreases, and the laminar flame thickness increases, which indicates the decrease of hydrodynamic instability. The ratio of unstretched laminar burning velocity with and without diluent is only related to the dilution ratio and is not influenced by the equivalence ratio. A linear correlation is found between the ratio of unstretched laminar burning velocity with and without the diluent and the dilution ratio.