This experiment was conducted to investigate the effect of mannanoligosaccharide as an immunostimulator with different protein levels on the growth performance, proximate nutrient digestibilities, apparent amino acid digestibilities and immune status in pigs weaned at 21 days old. One hundred-fifty three way crossbred (Landrace x Large White x Duroc) pigs averaged 6.08kg of body weight, and weaned at 21 days of age were assigned to six treatments by a 2 x 3 factorial arrangement [two level of MOS (0 and 0.1%) and three level of protein (20%, 23% and 26%)]. Each treatment has 5 replicates with 5 pigs per replicate. The pen is an experimental unit. Pigs fed diet with MOS had improved weight gain and feed consumption (p$lt;0.05) during the entire period, however, the protein level did not improved growth performance, and no interaction between the protein and MOS levels was observed. Although there was no significant difference, pigs fed the diet with high (26%) or medium (23%) protein showed better average daily gain (ADG) and feed efficiency than pigs fed diet with low (20%) protein regardless of MOS addition. The protein level did not affected the nutrient digestibilities of experimental diets. And, also, there was no interaction between protein level and MOS addition in digestibilities. Only dry matter digestibility was significantly higher in pigs fed the diet with MOS than pigs fed diet without MOS (p$lt;0.05). There was no main effect of protein level and MOS addition, and no interaction effect between protein level and MOS addition in essential and non-essential amino acid digestibilities. Protein level or MOS addition did not affected immune parameters in pigs. Additionally, no interaction effect on immune parameters was observed between protein level and MOS addition. However, pigs fed diet with the medium (23%) protein showed lower CD4+:CD8+ ratio than pigs fed other diets regardless of MOS addition, and pigs fed diets with the MOS showed low immune status such as low CD4 positive lymphocytes, high CD8 positive lymphocytes, low CD4+:CD8+ ratio during the overall period without considering protein level. In conclusion, the pigs fed diets with the immunostimulator like MOS improved growth performance and maintained low immune system when compared to pigs fed diets without the MOS. However, there was no significant difference in growth performance and immune status between protein levels.