Abstract: Laserinduced fluorescence (LIF) spectrum technique is introduced to study combustion process. The experimentation for LIF spectrum of small molecules common in combustion process is described and its results are given. Experimental schemes are designed to measure the spectrum of OH and NO freeradicals by the technique of LIF. OH freeradical in alcohol burner and candle flame and NO freeradical in B/KNO3 pyrolyzing and laser ignition are measured by instruments of YAG laser, dye laser, CO2 laser, spectrometer and ICCD. Experiment results show that fluorescence spectrum is independent with excitation wavelength, but the fluorescence intensity may be decreased when the excitation wavelength changes because the fluorescence deviates from optimal wavelength. The experiment shows that the results agree with fluorescence spectrum characteristics. In comparison with other techniques, the LIF spectrum technique possesses the advantages of superior selectivity and sensitivity.