Abstract: Soil nonlinearity around the pile becomes significant under lateral dynamic loading. Several laboratory model tests were conducted to investigate the nonlinear behavior of laterally loaded piles with different length-diameter ratios in sand, and to analyze the influence of loading amplitude and pile's length-diameter ratio on the nonlinearity. Test results show that the higher load amplitude leads to much stronger pile-soil nonlinear interaction and lower resonant frequency of the pile-soil system. The resonant frequency of the pile-soil system as well as its attenuation rate is negatively correlated with the pile's length-diameter ratio in case of the same pile diameter.