This research is the empirical study that the effects of pre-roll ads, recently being interested in the field of advertising, are revealed. The previous researches were about the advertisements that were expose after programs, however, this study examined the effects of pre-roll advertisements that had been adopted from YouTube and Naver TV Cast. Specifically, this study verified effects of pre-roll advertising which are composed of degree of engagement of the video mobile contents and the length of advertising, and moderating effect of intrusion, The results showed that the higher engagement of video contents, the lower effects of pre-roll advertising. Also, the group exposed in the 30s’ pre-roll advertising was higher in the recall than the group exposed in 15s’ one. But, there was not found in the effect on attitude toward advertising. Lastly, this study revealed the moderating effect of engagement of video contents on attitude toward advertising.