Abstract: The granularity, density, humidity and structure of soil are the natural indexes of its components and state behaviors, which can be used to describe its mechanical property and variation law. The structural index is a quantitative parameter of structure. It is related with the granularity, density and humidity of soil components and state behaviors and has an essential relationship with the fabric arranged with soil particles and the connection between soil particles. Both the fabric and connection have the steadiness to resist external loads as well as the variability under damage loads. The structural index is established from the steadiness and variability of loess structure in terms of the product of the ratio of intact to remolded sample unconfined compressive strengths (UCS) and the ratio of intact to saturated sample UCSs. It essentially covers water-immersion and load-disturbance sensitivities, and reveals their variation laws. By analyzing the relationships between the structural index and the difference between natural water content and plastic limit, liquid limit and liquidity index, the essential relations of the structural index with the granularity, density and humidity are revealed. In addition, a composite physical index in terms of liquid limit, dry density and void ratio is proposed to study its influence on the structural index. The results show the structural index of loess has a monotonic relation with the composite physical index under approximate dry densities. Thereby, the reliability and reasonableness of the structural index of loess are further verified.