Objective This study was designed to investigate the effects of Gamiygin-tang (GY) extract (GYE) and fermented solution (GYF) on body weight, serum lipid level and adipocyte differentiation in high fat diet-fed obese mice. Materials and Methods High fat diet-fed obese mice and 3T3-L1 adipocytes mice were treated with GYE and GYF and obesity related markers were assessed. A cytotoxicity assay was carried out by MTS assay. Inhibitory effects of GYE and GYF on adipocyte differentiation were carried out by Oil Red O staining. The effects of GYE and GYF on the expression of adipocyte differentiation regulatory factors, peroxisome proliferator-activated receptor gamma ($PPAR{gamma}$) and CCAAT/enhancer binding protein alpha (CEBP-${alpha}$) were measured by real-time reverse transcriptase-polymerase chain reaction. The effects of GYE and GYF on the expression of adipocyte differentiation regulatory factors were also determined in relation to protein production/protein levels by western blotting. The anti obesity effects of GYE and GYF were measured in high fat-diet induced obese mice. Various factors were measured from the serum of the high fat-diet mice. Results Though GYE did not show toxicity at the concentration of 1mg/ml, GYF showed toxicity at the concentration of 1mg/ml. The GYE at 0.1 and 1mg/ml inhibited the differentiation of 3T3-L1 cells, and the GYF also inhibited the differentiation of 3T3-L1 cells. The effect of GYE on adipocyte differentiation factors including PPAR-${gamma}$ and CEBP-${alpha}$ was investigated and compared to the corresponding concentration levels of GYF. GYE and GYF both suppressed the RNA and protein levels of adipocyte differentiation factors. In the animal test both GYE and GYF reduced weight gain. GYE and GYF reduced blood cholesterol, TG and LDL levels. GYF better reduced blood cholesterol levels. Conclusions These results demonstrate that GYE and GYF exerts anti-obesity effect in 3T3-L1 cells and obese mice induced by high-fat diet.