Eighteen lactating sows (Landrace × Yorkshire) were used in an experiment to determine the effects of fermented garlic by Leuconostoc mesenteroides KCCM35046 on growth performance, blood constituents, nutrient digestibility, fecal microflora, and fecal scores in sows and their piglets. The experiment was started before farrowing 30 d to weaning, and sows were randomly assigned to the following dietary treatments: control basal diet (CON) and the CON + 0.1% or 0.2% fermented garlic (FG). During the lactation period, sows had free access to feed and water. At farrowing day and weaning (day 28), body weight (BW) loss of sows reduced linearly (P = 0.042; P = 0.006, respectively) in FG diets. There was a linear improvement in piglets’ BW with increasing dietary FG levels at weaning (P = 0.033). Piglets fed the diets with the FG linearly increased the average daily gain (ADG) at the 3rd and 4th weeks (P = 0.041; P = 0.002, respectively). Overall, the ADG of piglets linearly increased when lactating sows fed the diets with FG supplementation (P = 0.004). Red blood cell concentrations linearly decreased (P = 0.004) in sows fed dietary FG supplementation at the weaning day. Dietary inclusion of the FG tended to increase the fecal Lactobacillus population and decrease Escherichia coli population of piglets at weanling day (P = 0.075; P = 0.051, respectively). In conclusion, the results of this study indicated that dietary supplementation of FG (0.1% and 0.2%) improved growth performance in sows and their piglets.