The usability of a website refers to the ease of the system use and the ability of the users to complete their tasks efficiently and without error. Websites should be user-friendly while designed to be more multi-functional in conjunction with the increasing user necessities. Herein, understanding website usability-related factors becomes important in designing such usable systems. In the current study, we aim to provide a systematic approach to categorizing website usability factors by referencing the usability problems encountered through a cognitive walkthrough approach. In this study, first, the usability problems of a specific website were identified by three experts and the problems were rated according to the severity levels, from cosmetic problems to catastrophic problems. Then, usability problems were mapped with the website usability factors. In the last step, the categorization of website usability factors according to the severity levels was conducted with a cross-tabulation approach, correspondence and cluster analyses. As a result, Individualization and User Adaptation(IND), Privacy and Business Policies(PRV), Link Design(LNK), Search(SRC), and Help(HLP) were grouped with the severity level of catastrophic problems. These factors are the most significant factors that website developers should think about first in the design and evaluation of websites.