This study examined 38 EFL senior high school students' "Yahoo!" online dictionary look-up behavior. In a language laboratory, the participants read an article on a reading sheet, underlined any words they did not know, looked up their unknown words in "Yahoo!" online bilingual dictionary, and wrote down the definitions of their looked-up words. The participants' dictionary look-up records were collected, their look-up errors were categorized into four types, and their look-up behavior was interpreted through seven patterns. The results showed that some participants looked up a word in the dictionary without removing the inflection of it, looked up individual words instead of a fixed expression, did not make good use of the example sentences or phrases provided by the dictionary, or did not take the context into consideration when selecting an appropriate meaning for a word. Based on the students' online dictionary look-up errors and their dictionary-consulting behavior patterns, pedagogical implications concerning dictionary look-up instruction are raised and discussed.