This research aims at: (1) improving the students’ writing skill through Collaborative Writing; (2) identifyingthe problemsin the implementation ofCollaborative Writing.
The method used in this research is classroom action research. The research was carried out to the tenth grade students of SMA Negeri 2 Karanganyar.  The research data were collected using qualitative technique and quantitative technique. Technique of collecting data using qualitative technique can be divided into two categories: (1) observational (observation, diary, audio recording and transcription). (2) non-observational (interview, document and questionnaire). While technique of collecting data using quantitative technique was obtained from the writing test that consists of pre-test and post-test. The qualitative data  were analyzed by using constant  comparative techniques that consists of assembling the data, coding the data, comparing the data, building interpretations and reporting the outcomes. While the quantitative data were analyzed by comparing the students’ mean score of pre-test and post-test by using Descriptive Statistic technique and also using t-test to test the significant differenceof the students’ pre test and post test.
The research findings show that: (1) the use of Collaborative Writing improves the students’ writing skill. It can be seen from the increasing of the mean of the students’ score.  Collaborative writing provides activities particularly discussion section and pair-checking section which facilitated students to improve their writing skill; (2) in implementing  Collaborative Writing time allocation and students’ communication in group needs to be considered.This might imply that besides having a good knowledge and subject mastery, the teacher should have good management capability both in time management and class management

Keywords: Collaborative Writing, writing skill