Lactobacillus reuteri as probiotic helps prevent and treat both viral and bacterial diarrhea enhancing the body's resistance to gastrointestinal disease. However, the growth of L. reuteri is very limited and certain nutrients are needed for optimum growth and high cell density. The objectives of this study were to determine the effect of different protein sources on growth and bacterial cell density of L. reuteri and to assess the optimum growth requirements using response surface methodology. Four strains (CF 2‐7F, MF 14 C, DSM 20016, and SD 2112) of L. reuteri were used in this study. Overnight cultures were inoculated into fresh basal growth broth with various proteins sources (peptone, tryptone, proteose peptone, phytone peptone, tryptic soy broth, yeast extract, and beef extract). Samples were then mixed well and incubated at 37 °C for 48 hrs. Bacterial growth was monitored by measuring turbidity at 610 nm in a spectrophotometer at different time intervals during the incubation period. At the end of incubation, samples were plated on MRS agar to determine the bacterial population. Our results showed that phytone peptone or tryptone promoted the growth of L. reuteri (P < 0. 01) compared to the control. The response surface model indicated that phytone peptone (5–10 g/L), tryptone (5 g/L), and glucose (15g/L) could be used as optimum concentrations to enhance the growth and cell density of L. reuteri in growth medium.