ABSTRACTThis study aims at: (1) describing the implementation of authentic assessmentin English instruction based on curriculum 2013, (2) figuring out the difficulties inimplementing authentic assessment in English instruction based on curriculum 2013and (3) yielding the solution used by the teacher to overcome the difficulties inimplementing authentic assessment in English instruction based on curriculum 2013.This study applies a case study method which had been carried out at thetenth grade of SMA Tunas Bangsa, Aceh Barat Daya in the academic year2014/2015.The data were collected through interview, observation anddocumentation and validated by using triangulation and member checking method.The data were then analyzed by using the interactive model of Miles andHubberman.The result of the study revealed that SMA Tunas Bangsa Aceh Barat Dayautilized Permendikbud 81a Tahun 2013 as the primary guideline in implementingauthentic assessment in curriculum 2013. Particularly, the English teacheradministered oral monolog test as the performance assessment, descriptive writtentest as the portfolio assessment, and assigned students to produce a tourism brochureas the project assessment. However, the complex assessment procedure, managingthe classroom assessment activities and scoring the students’ tasks were the maindifficulties in enacting this assessments. To overcome the problems, the teachermaintained annual communication with students concerning the assessment system,organized the tasks orderly and adjusted the scoring instruments to the level ofstudents’ English proficiency and the school condition.SMA Tunas Bangsa Aceh Barat Daya has been implementing the authenticassessment as directed in curriculum 2013. However, the English teacher stillexperienced some constraints during instructional activities so that the assessmentprocess did not run effectively. Therefore, the English teacher is expected to extendher insight, creativity and dedication to improve the quality of the assessment. Thecurriculum coordinator is recommended to increase the coordination betweenteachers to create more effective assessment management.Keywords: implementation, authentic assessment, curriculum 2013