The aim of the research conducted in order to find out: (1) whether there is any significant difference in writing achievement between the student taught using Task Based Instruction and Group Investigation (GI) method; and (2)which method has a better result in writing, the group taught using Task Based Instruction or using Group Investigation (GI) method. The method used within this research is a quasi-experimental design. This research was conducted at Senior High School in Surakarta in the academic year of 2018/2019.  The population of this research is the eleventh grade of Senior High School in Surakarta. The samples of this research are the XI IPA 3 as the experimental class which consist of 28 students and XI IPA 1 as the control class which consist of 28 students. The instrument used to collect the data within this experiment is test. The data were analyzed using the t-test formula. The calculation of the t-test data shows that t observation (t?) = 2.02 is higher than t table (54, 0.05) = 1.960. Thus it can be concluded that there are differences in writing achievement between the students taught using the Task Based Instruction (TBI) and those taught using the Group Investigation (GI). The mean score of the students taught using the Group Investigation (GI) is 77.64, meanwhile the mean score of the students taught using the Task Based Instruction is 75.80. Therefore it can be concluded that the students taught using the Group Investigation (GI) have a higher achievement score than those taught using the Task Based Instruction (TBI).So, it can be concluded that Group Investigation (GI) is more effective to teach writing than Task Based Instruction (TBI).

Keywords: Task Based Instruction (TBI), Group Investigation (GI), Writing Skill.