The influence of vibration pressure and frequency on the mechanical properties and stress-relaxation was investigated via stress-relaxation test and tensile test. First, it had been observed in the tensile test that the tensile fracture elongation reached the maximum at 20 Hz for polypropylene (PP) and 15 Hz for polypropylene/calcium carbonate (PP/CaCO3), respectively. With the increasing vibration pressure, the tensile fracture elongation would decrease. Second, the dynamic mechanic analysis has been used to test loss angle tangent value of the material. After the dynamic mechanic analysis, the simples have been installed in the universal tensile testing machine which applies the 2% strain on the simples. From these experiments, it has been discovered that the trend of the changes of stress-relaxation is similar with the trend of the changes of loss angle tangent value. When the vibration frequency reaches the 20 Hz for PP and 15 Hz for PP/CaCO3, the stress-relaxation is larger than that of other materials prepared at the same pressure (10 Mpa); meanwhile, the stress-relaxation of these materials, which has been prepared at the same frequency (10 Hz), decreases with the increasing vibration pressure. According to above tests, it is also very useful to improve the stress relaxation properties via changing the condition of the vibration field. POLYM. ENG. SCI., 49:454-461, 2009. (c) 2008 Society of Plastics Engineers