In the increasingly competitive marketplace, the goal firm's communication is to convince consumers that the advertised brand is superior to some alternative brand. Comparing the advertised brand with a referent brand helps attract consumer attention to the advertised brand. Lately, many comparative advertising researchers consider mediating variables alternative to find the reason for the different results of comparative advertising effects. This study considers endorser and competitive brand interference as mediating variables which may affect the comparative advertising effects. Independent variables of this study are the comparative advertising types(direct/ non-comparative), competitive brand interference(interference/non interference), and endorser type (celebrity/ non-endorser). Dependent variables of this study are consumers' attitude towards the ad, brand attitude, and product purchase intention. This study was run as a 2×2×2 factorial design with 30 subjects per cell, resulting in a total sample size of 240. This study randomly assigned each subject to 1 of the 8 treatment groups. The result of this study shows that there is no significant difference between the comparative advertising and non-comparative advertising in terms of their effects. And this study found that competitive brand interference was important variable to mediate comparative advertising effectiveness. The findings of this study illustrate the need to consider competitive brand interference as an aid to the executions of effective comparative advertising.