This study aims at: (1) describing the application of authentic assessment of writing (2) identifying the teacher’s reasons on the application of authentic assessment of writing (3) identifying the problems faced by the teacher and students on the application of authentic assessment of writing and (4) finding solutions offered by the teacher to overcome the problems on the application of authentic assessment of writing. This study was conducted at a high school in Central Java in 2017/2018 academic year. It is a qualitative research in the form of case study. The subjects of this study were an English teacher and 11th  grade students. The primary data were taken from an interview.
The findings show that: (1) the teacher has applied authentic assessment of writing on the aspects of attitude, knowledge and skills competency by applying several procedures namely identifying standards, determination of authentic tasks, making criteria and making rubrics (2) the teacher had several reasons in applying authentic assessment, namely authentic assessment is a curriculum 2013 demand, authentic  assessment  is  able  to  be  correlated  to  the  real-world  context  and authentic assessment is suitable to be developed for writing assessment (3) the teacher and students found problems in applying authentic assessment. The teacher’s problems are the understanding of authentic assessment, time limitation and students’ writing ability. In line with the teacher, students also faced problems in their writing ability, limited time and complexity tasks (4) the teacher applied several solutions to overcome the problems, including discussions with other teachers and vice principal of curriculum, selecting the effective and efficient assessment and time management effectively.
This study is expected to give some implications. It provides insight for the development of authentic assessment of writing in teaching-learning English. Therefore, the teacher knows the types, techniques, instruments, procedures and the reasons for carrying out authentic assessment. Besides that, the teacher is able to provide accurate solutions to solve the problems faced by both the teacher and students. So that, it can be a motivation to apply and develop authentic assessment in order to achieve the goals of a learning.