Abstract: Objective To observe the effects of oral probiotics supplementation (Lactobacilluscasei strain Shirota, LcS) on inflammatory response, upper respiratory-tract infection, and immune function of soldiers in military training. Methods This was a randomized controlled trial. We recruited 100 male volunteers from soldiers in Chinese PLA General Hospital. The volunteers were randomly divided into experimental group (n=50, received 100 ml of fermented milk beverage containing LcS twice a day) and control group (n=50， received 100 ml of ordinary milk twice a day). The dietary intake, upper respiratory tract infection and gastrointestinal reaction were investigated and the NK cell activity, serum inflammatory factors, and immune indicators and saliva IgA levels were measured at 0, 2 and 4 months, respectively. Results A total of 87 subjects completed the intervention (45 in experimental group and 42 in control group), with the lost to follow-up rate of 13%. At 2 months, the levels of serum lgG, CD3 and CD4 were significantly higher in the experimental group than those of control group IgG, (1 268.24± 290.81) mg/dl vs (1 124.53± 272.79) mg/dl; CD3, (0.72± 0.07) vs (0.66± 0.05); CD4, (0.38± 0.07) vs (0.35± 0.05); all P ＜ 0.05; At 4 months, the levels of serum lgE, lgG, CD3, CD4, CD4/CD8, saliva lgA, neutrophils and lymphocytes were significantly higher in experimental group than control group lgE, (231.14± 177.71)mg/dl vs (142.74± 112.08) mg/dl; lgG, (1 243.22± 290.58) mg/dl vs (1 036.23± 228.71) mg/dl; CD3, (0.70± 0.05) vs (0.65± 0.04); CD4, (0.41± 0.06) vs (0.34± 0.04); CD4/CD8, (1.68± 0.43) vs (1.31± 0.32); saliva lgA, (181.67± 39.12)μ g/ml vs (147.38± 33.53)μ g/ml; neutrophils, (0.57± 0.06) vs (0.53± 0.08); lymphocytes, (0.53± 0.08) vs (0.33± 0.06); all P ＜ 0.05, while serum C-reactive protein, IL-1 level and white blood cell count were significantly lower in experimental group than control group serum C-reactive protein, (0.31± 0.02) mg/dl vs (0.51± 0.25) mg/dl; IL-1,(9.96± 4.44) pg/ml vs (14.51± 6.67) pg/ml; white blood cell, (5.10± 0.35)109/L vs (7.12± 0.93)109/L; all P ＜ 0.05; The incidence of gastrointestinal disease in experimental group was significantly lower than that of control group (2 months, 4.45% vs 23.80%; 4 months, 4.45% vs 21.43%; P ＜ 0.05, respectively); The incidence of upper respiratory tract infection in the two groups showed no significant difference (2 months, 8.89% vs 11.90%; 4 months, 8.89% vs 11.90%; P ＞ 0.05, respectively), but it showed a downward trend. Conclusion Long-term oral LcS supplementation can reduce the inflammatory response and improve the immune function, which may be beneficial to preventing upper respiratory tract infection.