Natural loess in the northwest of the Loess Plateau was sampled, measured and studied for aggregate amount, relationship between the aggregates and CaCO₃ dissolution, composition of the aggregates, and CaCO₃ distribution on the surface and in the interior of the aggregates with aid of Scanning Election Micrograph (SEM).The results show that: CaCO₃ takes part in the formation of almost all aggregates; the bigger the aggregates are, the higher the CaCO₃ amount is.whereas the number of aggregates is inversely proportional to CaCO₃ dissolution amount.With decrease in CaCO₃ distribution on aggregate surface, the amounts of microaggregates, silts and clay attaching to the aggregate surface reduce.Inside the aggregate, there are three types of cementation between CaCO₃ and mineral particles: CaCO₃ is adsorbed to mineral particle surface, CaCO₃ distributes among mineral particles and mineral particles attach to CaCO₃ surface.The experimental results testify that the main cement of loess aggregates is CaCO₃.