Soil aggregates are the basic units of soil structure and play an important role in soil quality and fertility. It is thus meaningful to investigate the natures and differences in biochemical properties of different soil aggregates. Moreover, it may be benefit to understand the essence of soil fertility and fundamental reason for the difference in fertility of different soils. In this study, 4 surface soils were sampled from woodland, none-irrigated cropland, irrigated cropland, and grassland in semi-arid region of Inner Mongolia. Six classes of soil aggregates with different diameters were collected with dry sieving method. The measured parameters included soil basic properties such as clay, organic matter, total nitrogen, K₂SO₄ extracted carbon, and soil microbial biomass C (MBC), ATP, basal respiration and substrate-induced respiration. The results showed that the irrigated soil had highest clay and organic matter contents, about 32.2 g/kg and 29.93 g/kg respectively, but lowest values in grassland soil, only 12.2 g/kg and 12.48 g/kg. The irrigated soil contained microbial biomass C of 336.03 mg/kg, but the woodland soil and none-irrigated cropland soil had about 207.53 mg/kg and 216.11 respectively. The ATP content in soils ranged 0.0016 - 0.0031 μmol/g soil, higher in the irrigated soil, lower in the woodland soil. The soil microbial biomass ATP concentration was the highest in the irrigated soil, about 9.26 μmol ATP/g C_mic, but lowest in the woodland soil, only 7.79 μmol ATP/g C_mic. The soil basal respiration and substrate-induced respiration were the highest in the irrigated soil, 10.31 μg CO₂-C g^-1soil d^-1 and 132.2 μg CO₂ g^-1 d^-1 respectively, but very low in the no-irrigated soil, only 3.33 μg CO₂-C g^-1soil d^-1 and 88.66 μg CO₂ g^-1 d^-1. The microbial biomass metabolic quotient (qCO₂) was high in the irrigated soil, about 0.69μg CO₂-C g^-1soil d^-1 but very low in the none-irrigated cropland soil, about 0.35 μL CO₂-C g^-1soil d^-1. The result showed that the microbial activity in the irrigated soil was higher than other soils, which might be linked with the quantity and quality of organic C input, water soluble carbon content and water content. Different soil aggregates had different natures of microbial biochemical properties. The basal respiration, qCO₂ and substrate-induced respiration in different soil aggregates were 0.29-0.71 μL/g C_mic, 0.29-0.71 μL/μg C_mic and 64-120 μg CO₂/g soil, respectively. The ATP content in soil aggregates changed among 0.83-3.02 nmol/g soil. The soil microbial biomass ATP concentration in soil aggregates were fallen within the range of 6.54-9.32 μmol/g C_mic. In general, the larger the soil aggregates, the higher values of those biochemical parameters were measured, which may imply the difference in microbial community and physiological activity among the soil aggregates. It may also indicate that microorganisms in larger aggregates (> 0.25 mm) have higher carbon utilization efficiency than those of smaller aggregates (< 0.25 mm). Soil microbial biomass ATP concentration, soil basal respiration and substrate-induced respiration were significantly and positively correlated with the soil microbial biomass C(r=0.929, 0.865, 0.885, P < 0.01). There was also a significant correlation between basal respiration and substrate-induced respiration (r=0.828, P < 0.01). It is evident that the relationships among the soil microbial parameters does not change with soil aggregates though dramatically different in the absolute values.