We investigated the effects of dietary supplementations of yeast (YFA) or non-yeast fermented additives (NYFA) at 5.0 and 10 g kg(-1) on the performance parameters and the size of digestive organs of broiler chickens. An isocaloric and isonitrogenous basal diet used as negative control was added with 5.0 or 10.0 g kg(-1) of YFA or NYFA. Each of five experimental diets was given to broiler chickens kept in three randomized floor pens, each with 21 birds, from one-day old to 42 days old. Although the feed intake of broiler chickens was not significantly influenced by the diets, there were significant increases in weight gain and improvements in feed conversion ratio and carcass yield in broiler chickens fed the diets supplemented with 5.0 and 10 g kg(-1) of YFA and NYFA. The rate of improved weight gain and feed conversion ratio by the dietary treatments was proportional to their viable yeast counts. The diet containing 10 g kg(-1) of NYFA was not recommended due to remarkably reduced carcass yield as a result of increased weights of digestive tract. The remaining dietary treatments generally caused a reduced length and relative weight of the digestive tract. Of the dietary treatments, the diet containing 10 g kg(-1) YFA caused a significant increase in digesta viscosity with the diet. Significant growth-promoting effects were obtained from the dietary supplementations of 5.0 g kg(-1) NYFA or 5.0 and 10 g kg(-1) YFA.