The aim of this study was to evaluate the effect of Lactobacillus plantarum and fructooligosaccharides on weight gain, change the pattern of enteric fermentation and intestinal inflammation in preterm infants in the neonatal intensive care unit in Kamali Hospital of Karaj. Inclusion criteria: all premature infants hospitalized in Kamali Hospital of Karaj and exclusion criteria: in the case of clinical symptoms of Sepsis (including severe lethargy, inability to feed, frequent vomiting, impaired consciousness, seizures, severe abdominal distention , laboratory findings such as positive blood culture and elevated CRP) or critically ill patient for any reason. After determining the population under study, infants were randomly divided into 4 groups: Symbiotic group: this group will received 5 drops of milk fortified with 108 Colony Forming Unit per ml (cfu/ml) of Lactobacillus plantarum and 0.8 grams per dL fructooligosaccharides twice a day for two weeks, probiotics group: this group will received 5 drops of milk fortified with 108 cfu/ml of Lactobacillus plantarum twice a day for two weeks , prebiotic group: this group will received 5 drops of milk fortified with 0.8 grams per dL fructooligosaccharides twice a day for two weeks, control group: this group will received regular milk twice a day for 2 weeks. After two weeks, anthropometric indices including height and weight of babies will be recorded. Also biochemical markers of colonization of probiotic bacteria include acetate, lactate and calprotectin in the stool of infants will be measured. The results of each group will be compared with control and statistically analyzed.