The loess soil distributed in north Mongolia, Orkhon-Tuul river basin area. Average thikness of loess-soil formation is about 1-2 meter and some places reaches up to 10-20 meter. On the base of radiocarbon (C14) analysis data attempting to define loess-soil formation age and specifics. Total 6 soil profiles with 1-6 meter depths studied and C14 analysis done for 14 buried soil samples. Used linear and nonlinear function for the study of sediment accumulation spesifics. In the steppe plain of Orkhon-Tuul river basin area top 1 meter thick loess soil formation start to accumulate from 3600-4000 years before present. More deep (1-5 meter) loess sediments accumulated from 9000 years BP till 3600-4000 year BP or Holocene optimum time. Average silty loess material accumulation rate for 1 meter depth topsoil about 40,9-47,9 year for 1 cm of layer, and for 1-5 meter deep layers about 24.8-28.7 year for 1 cm of layer. Early Holocen periods loess soil formation was two times active than late Holocen period. Probably climate condition of Early Holocen period was more dry and less vegetations.
