Abstract: Paper introduces a technique to improve the vacuum evaporation methodby intalling an additional quartz micro-balance to control the thickness ofsources, proposing a new procedure of vacuum evaporation to avoid contaminationrisk, reducing some factors effecting source uniformity and simplifing theequipment. This improvement makes the method of preparing sources by vacuumevaporation quite simple, safe and reliable. Using this improved method, somefirm, uniform and thin internal conversion sources of ~(170)Tm with high resolution(0.05% for the 75.3 keV peak of L line) are obtained.