Mixed ability class is an important fact of all language classes, no matter where do teachers live in the world or which educational institute they teach. In a mixed ability language class, most of the language teachers often face the challenges of having wide range of language proficiency levels of the students. Not only the teachers, but also the students feel frustrated and demotivated in a mixed ability class. This paper is an endeavor to identify some problems of teaching writing skill in mixed ability classes at tertiary level in private universities in Bangladesh. This study aims at finding out the advantages of mixed ability classes. This paper tries to find out the teaching strategies applicable for mixed ability writing classes. Finally some recommendations have been suggested for the mixed ability writing classes.