Our study reported the effect of a synbiotic containing L. paracasei and O. humifusa on the improvement of stool consistency; additionally, we also provided scientific evidence of the mechanism underlying the beneficial effects of the synbiotic using IBS-related factors reported in previous studies, including the decrease in the serum corticosterone levels, low levels of TNF-α in the colonic mucosa, and increase in the expression of the tight junction proteins. The gene discussed is TNF; the disease is irritable bowel syndrome.