Abstract: In order to boresight the sensors in a multi-spectral electro-optical tracker, three methods for measuring the optical parallelism between laser and infrared sensor were introduced and compared. The boresight method using the thermal target to transform laser wavelength into infrared wavelength were investigated. The investigation shows that the selection of the thermal target and the measurement of laser transmittance are the most important steps. Based on the experiment result, the phenolics was selected as the material for thermal target. Its laser transmittance is only 0.5%.The result obtained by this method conforms with the result obtained by long range target method, its feasibility and validity are proved.