This study aimed at investigating the relationship between types of dictionaries used and lexical proficiency in writing. Eighty TOEFL students took part in responding to two Questionnaires collecting information about their dictionary type preferences and habits of dictionary use, along with an interview for further in-depth responses. They were also asked to write a composition to be evaluated on their lexical proficiency. According to the results, the most frequently used dictionary type was Bilingual (E-P) Desk Dictionary (BDD) and the least favored dictionaries were Bilingual (P-E) Desk Dictionary (BDD2) and Bilingual (P-E) Mobile Dictionary (BMD). Referring to dictionaries in search of meaning during reading-writing, and after speaking-listening were the most common habits of students. Additionally, a significant relationship between the type of dictionary and lexical proficiency in participant's writing was found. There was also a correlation between skill in dictionary use and lexical proficiency in writing. These findings entail some pedagogical benefits to enhance learners' lexical competence by modifying the type of learners' preferred dictionary and their habits in dictionary use.