Based on the principles of radio-frequency identification (RFID) technology and Friis propagation equation, the path loss expression of ultra high frequency (UHF) RFID in free space is provided. The Fresnel clearance and horizontal interval between reader antenna and tag are employed as dependent variables, and then the obstructing effect of the first Fresnel zone on path loss is discussed. By the methods of linear regression and minimum mean-square error, a dual-slope Logarithm distance path losses model is proposed. The path losses of UHF RFID under different parameters are measured in open indoor environment. The measurement results indicate that RFID system experiences less fading when the Fresnel clearance is 1.5 times higher than the first Fresnel radius. The standard deviation of the proposed model with two slopes reduces ten percent or more compared with that of traditional logarithm distance path loss model.