The research aims to describe the application of authentic assessment for writing skill in English learning process. Authentic assessment is a form of assessment that requires students to demonstrate the competence of attitude, knowledge and skill gained from the learning process that linked to the real-world context. The research was conducted at senior high school in Indonesia. It is a qualitative research in the form of qualitative descriptive approach. The subjects of the study were the English teacher and the students of grade eleventh. The data were taken from observation, interview, and documentation. The findings show that the teacher has been implemented authentic assessment for writing skill. The teacher used several techniques and instruments in applying authentic assessment for students writing skill: (a) attitude assessment (observation and journal), (b) knowledge assessment (written test and assignment), (c) skills assessment (project and product assessment). But in its application, teacher still found some various problems. They were time limitation of learning and complexity of assessment procedures. Nevertheless, teacher made several efforts to overcome the problems.