This paper examines the antecedents and consequences of product uncertainty in online marketplaces by conceptualizing the dimensions of product uncertainty - description uncertainty (identifying product characteristics), performance uncertainty (inferring product"s future performance) and fit uncertainty (matching product"s characteristics with buyer"s needs), with the focus on product fit uncertainty. It also theorizes the distinction, relationship, and effects of the three dimensions of product uncertainty. Finally, it proposes a set of IT artifacts to reduce product fit uncertainty. The hypotheses are tested with survey and website transaction data from 274 buyers in Taobao, the largest online marketplace in China. The results first demonstrate the distinction between three dimensions of product uncertainty, show that relative to description and performance uncertainty, only fit uncertainty has significant effect on price premiums, satisfaction, product returns, and repurchase intentions, and support the effects of the use of IT artifacts, such as instant messenger, product forums, and decision support tools on reducing fit uncertainty. Implications for research, theory and practice are discussed.