Although fluency is included as one of main aspects to teach and assess in the course of "English Writing" for special-purposed high schools in Korea, any concrete and detailed idea has not been suggested in the guidelines to the course on how this concept could be understood and dealt with both in teaching and testing. This study, therefore, aims at exploring the concept of fluency in writing through both reviewing literature on fluency which is mainly with regard to speaking skills and investigating characteristics in terms of fluency which are observed from 390 writing samples written by 195 Korean students at a foreign language high school. As a result of inductive and qualitative analysis of the data, specific features were observed with regard to quantity, coherence, cohesion and language level. It is suggested that these features could be considered when we trying to understand fluency in depth and establish a rating scheme for it in the context of high school students" writing assessment.