Despite the fact that there exists a great deal of previous studies regarding uncertainty under e-Commerce environment, they seem to simply focus on considering trust as a factor which could reduce uncertainty and its effect. In this respect, previous studies about handling uncertainty issues in the process of e-Commerce seem to be mainly focused on measuring uncertainty about the e-Commerce seller or websites itself. Recently, studies about the uncertainty involved in e-Commerce have started to decompose the uncertainty into two categories such as product uncertainty and seller uncertainty. When the uncertainty is categorized into two types, it became easier for the researchers to identify those factors affecting the uncertainty. However, this study was initiated by the research motivation that it is not sufficient for the uncertainty to be divided into product uncertainty and seller uncertainty. Rather, it seems that we have neglected an important fact how decision makers perceive the uncertainty when they try to make decisions in the process of conducting e-Commerce activities. In one words, decision makers feel uncertainty throughout the whole phases of making decisions about the e-Commerce transactions, from initial phase to a final phase. Decision makers start to feel uncertainty when they started browsing e-Commerce websites to look for right products and sellers with reasonable price and trustworthy transaction conditions. They also feel uncertainty when they need to pay with credit cards. Even they feel uncertainty, to some degree, after they pay and wait for the arrival of products. However, there are few studies in literature about the uncertainty management in the fields of e-Commerce in which uncertainty is handled throughout the whole phases of decision-making. Therefore, this study proposes measuring uncertainty both in the pre-decision stage, and post-decision stage in order to reduce uncertainty and enhance QoD(Quality of Decision) with respect to the e-Commerce activities. Decision here implies an e-Commerce decision. Empirical results with valid 43 participants (21 for the pre-decision stage, and 23 for the post-decision stage) revealed that there exist significant differences in the level of perceived uncertainty between the pre-decision stage and post-decision stage. The level of perceived uncertainty for the pre-decision stage was higher than that of the post-decision stage. To be specific, we divided uncertainty into four dimensions- uncertainty about the brand of e-Commerce sellers (or websites), uncertainty about the product performance, uncertainty about the behavior of e-Commerce sellers (or websites), and uncertainty about the environment of e-Commerce sellers (or websites). Results were that the perceived uncertainty about the brand and behavior of e-Commerce sellers in the pre-decision stage was higher than that in the post-decision stage. Out of the empirical results like this, we could build a new conceptualization mechanism about uncertainty by using a mind-set theory showing that the cognitive structure and behavior in deliberative mind-set which refers to the state of mind before decision is different from cognitive structure and behavior in implemental mind-set which refers to the state of mind after decision. We hope that our results may be helpful for the researchers to develop a new theory to analyze the uncertainty embedded in e-Commerce activities.