The objective of this study was to determine the effect of a single strain direct-fed microbial (DFM, Lactobacillus acidophilus 1 × 10⁹ cfu/g) on growth performance and carcass traits of growing-finishing pigs. Sixty pigs (30 gilts and 30 barrows) with initial BW of 44.4 kg ± 1.8 kg were allotted to 2 dietary treatments in a randomized complete block design with sex and initial BW as blocks. Experimental diets met the NRC 2012 nutrient requirements. The inclusion levels of DFM were 0.28, 0.24, and 0.20% during phase 5 (d 0 to 28), phase 6 (d 28 to 49), and phase 7 (d 49 to 70), respectively. Growth performance was measured for 10 wk. Blood samples were collected at wk 9 to measure TNFα and blood urea nitrogen (BUN). At wk 10, pigs were slaughtered at a local abattoir to evaluate carcass and loin quality including HCW, CCW, backfat thickness (1st, 10th, and last rib), loin weight, loin color (Minolta colorimeter), loin marbling score (1 to 10, NPPC scale), drip loss (48 h), loin eye area, and chemical composition. Data were analyzed using Proc Mixed of SAS with treatment and sex as fixed effects and initial BW block as a random effect. P value less than 0.05 was considered significant and less than 0.10 tendency. The ADG, ADFI, and G:F were not different between treatments. Backfat thickness of pigs with DFM was smaller (P < 0.05, 24.5 vs. 29.6 mm) at the last rib and tended to be smaller (P = 0.088, 30.1 vs 34.4 mm) at the 1st rib than pigs without DFM. Loin of pigs with DFM tended to be redder (P = 0.084, a* value for redness: 7.22 vs. 6.35) than pigs without DFM. Chemical composition, drip loss, TNFα, BUN, and intestinal morphology did not differ between treatments. Collectively, dietary supplementation of a single strain DFM, Lactobacillus acidophilus to grower-finisher feeds reduced backfat thickness of pigs without affecting growth performance and loin quality.