Effects of boron( B) on the serum biochemical index and microstructure of immune organs in male obese rats were studied.40 male SD rats( 3-month old) were divided into five groups: normal control group, high-fat-diet control group and boron supplemental group of low, medium and high dose, randomly. The normal control group were fed with normal diet, the other 4 groups were fed with highfat diet to establish the model of obesity for 8 weeks. The boron supplemental group of low, medium and high dose were supplemented 20, 40 and 80 mg B / L in drinking water for 90 d, respectively. At the end, the rats were anesthetized and bled. The blood were collected from right atrium to detected the biochemical indexes related to liver function, and the thymus and spleen were obtained to weighted and fixed, then the samples were made into paraffin sections, stained with hematoxylin-eosin( HE) stain, observed and measured the histological parameters of immune organs.Compared with normal control group, the Lee's index and abdominal fat rate, the level of serum low density lipoprotein cholesterol( LDL-c) and the thymus weight were significantly increased( P 0. 05) in 20 mg / L supplement groups of boron. Compared with the high-fat-diet control group, the Lee's index and abdominal fat rate, the level of serum Apolipoprotein B( apo B) and LDL-c, the thymus weight and index were significantly decreased( P 0. 05) in 80 mg / L supplement groups of boron. Under the microscope, compared with high-fat-diet control group, splenic nodule area was increased significantly( P < 0. 05), splenic periarterial lymphatic sheath, marginal zone and splenic cord were also thicker significantly( P < 0. 05), thymus medulla / cortex ratio decreased significantly( P < 0. 05), the cells arranged closely, vacuolar like structures were less in the thymus medulla of 20 mg / L and 40 mg / L supplement groups of boron. Microstructure of spleen and thymus did not change significantly in 80 mg / L supplement groups of boron.Supplementation of 20 mg B / L could decrease the level of serum apo B and LDL-c, and increase the level of serum HDL-c, and protect the liver function and immune organ of rat from damage caused obese by high fat diet.