The current study is an attempt to investigate the explicit or the implicit instruction of metadiscourse markers and the writing skill improvement. The participants of the study (N = 90) were female Iranian students at Kish Institute of Science and Technology. Two experimental groups were defined in this study: experimental group “A” which received the explicit instruction of metadiscourse markers and experimental group “B” which were taught implicitly based on Hyland’s (2005) classification of metadiscourse markers. Two instruments were employed in the study: a pretest and a posttest. To elicit the pertinent data, the participants were given a pretest of writing ability to investigate if the learners had knowledge regarding the correct application of ‎metadiscourse markers in their writing. After 8-session treatment, a posttest was administered to compare the participants’ performance in use of matediscourse markers. The findings of the present study indicated that there was a significant difference in the participants’ pretest and posttest writing scores with regard to the application of metadiscourse markers. The findings revealed that metadiscourse instruction had a positive effect on the learners’ writing. In addition, the results showed that both the explicit and the implicit instruction of metadiscourse markers significantly improved participants’ writing ability.