Abstract: A lab-sized test and evaluation system of petroleum products evaporation loss (PPEL) in petroleum products loading tank is built up with reference to field operation process and performance of PPEL. Taking gasoline for example, the vented gasoline vapor mole fraction C_H and its mean value C_H and the total evapo-ration lossage η in the entire course of loading gasoline are measured and calculated and compared with their theoretic minimum values C_H0, C_H0, and η₀, respectively. Some key effect factors on C_H(C_H) and η are analyzed and appraised. The results show that ratio λ of height of the exit of filling gasoline pipeline to tank height, initial vapor mole fraction C₀ in the tank gas space, speed U of filling gasoline, and temperature t of the system influence notably on C_H (C_H) and η, and the increasing grads of C_H with the effect factors at the so-designed tests are about 0.198, 0.167, 0.211 (h·m^-1), 0. 0189 (℃^-1), respectively. At the test of C₀=0 and λ=0.1, the mean vented vapor mole saturation ϕ′/ ϕ′_max is 0.5, which is lower about 0.25 than the ϕ′/ ϕ_′max which is total mean mole saturation at the whole tests of C₀ =0 and λ=0.8. At the whole tests of C₀=0, η=0.192%~0.661% and η₀ = 0.083%~0.229%, but at the test of C₀ =0.56 and λ=0.8, η=0.695%. So, the so-called PPEL is very seriously and harmfully, which will bring about hidden fire acci-dent, environmental pollution, waste and degradation of petroleum products, and so on. Last, some advice is given to lower PPEL.