Garlic has a long history to be used for medicine and food purposes. Black garlic, the fermented product of fresh garlic, is considered with better biological activities, such as antioxidant activity, and is developed as an increasingly popular functional food. Polysaccharides are the major components of fresh and black garlic, and immunomodulatory activity is one major pharmacological effect of polysaccharides. Therefore, chemical characteristics and immunomodulatory effects of polysaccharides from fresh and black garlic are investigated and compared in vitro for the 1st time, in order to reveal their molecular and pharmacological differences. It is demonstrated that the molecular weights of polysaccharides from the 2 sources and molar ratios of monosaccharides after acid hydrolysis are greatly variant. The effects of polysaccharides from 2 sources on RAW 264.7 macrophages functions, including promotion of phagocytosis, release of NO, and expressions of several immune-related cytokines (including interleukin [IL]-6, IL-10, tumor necrosis factor alpha, and interferon gamma), were different from each other. The results indicated that fresh garlic polysaccharide exhibited stronger immunomodulatory activities than that of black garlic. Moreover, it is revealed that fructan might be the bioactive component in garlic and it is indicated that during the fermentation treatment, fructan constituents of garlic has degraded, and basically no immunomodulatory effect can be found in black garlic polysaccharides.