Abstract: Laser induced fluorescence（LIF） method was used to detect the polycyclic aromatic hydrocarbons（PAHs） in laminar premixed ethane and propene flames to study the distribution of PAHs in flames and the effect of equivalence ratio on PAHs formation. The fluorescence spectra of PAHs in laminar premixed ethane and propene flames ranging from 300 nm to 500 nm can be attributed to the PAHs with one to four benzene rings（A1-A4）. In this study, the intensity of A1-A4 fluorescence were measured, and the peak of PAHs fluorescence spectra held at 500 nm. The intensity of PAHs fluorescence spectra increase with the heights above burner surface. The fuel rich flame is obviously in favor of A1-A4 formation and the fluorescence spectra intensity of A1-A4 increase faster at relatively high equivalent ratios. The normalized intensity of PAHs fluorescence spectra between ethane and propene are compared and find that the length and the saturability of carbon chain have a significant effect on PAHs formation.