The aims of this study were to determine the effects of konjac glucomannan (KGM), a water-soluble dietary fiber from Amorphophallu konjac C. Koch, on the cytotoxicity and DNA damage of fecal water-treated Caco-2 cells, a human colon adenocarcinoma cell line, and to compare these effects with those of inulin, oligofructose (FO), cellulose and no fiber diet. In addition, the possible mechanisms by which dietary fibers modulated the toxicity of feces were investigated. Seven-week-old BALB/c mice were randomly allocated to consume an AIN-93 diet that contained no dietary fiber (FF) or 5 % (w/w) KGM, inulin, FO, or cellulose for 3 weeks. Fresh feces were collected during days 18 - 21. Our results indicated that the survival rate (%) of fecal water-treated Caco-2 cells was similarly enhanced by each dietary fiber as compared with that of FF group, respectively. The inhibition of fecal water-induced DNA damage of Caco-2 cells was in the descending order of inulin> FO> cellulose, KGM group. Cellulose significantly exerted a bulk effect, while KGM, inulin and FO significantly increased the ratio of lactobacilli and bifidobacteria in feces, respectively, as well as the ferrous ion-chelating ability of fecal water, respectively. Therefore, this study suggests that KGM, inulin and FO may reduce the toxicity of fecal water mainly by increasing the fecal probiotics and ferrous ion-chelating activities while cellulose may act mainly by increasing the fecal bulk. FF: fiber free FO: oligofructose KGM: konjac glucomannan.