Penelitian ini bertujuan mengetahui pengaruh model pembelajaran kooperatif berbantuan asesmen portofolio terhadap kemampuan menulis dalam bahasa Inggris siswa dibandingkan dengan model pembelajaran konvensional. Populasi penelitian adalah seluruh siswa kelas VII SMP Negeri 1 Payangan berjumlah 287 siswa, sebanyak 84 siswa dipilih sebagai sampel dengan teknik random sampling. Data dalam penelitian dikumpulkan menggunakan instrumen tes kemampuan menulis dan kuesioner kecemasan selanjutnya dianalisis dengan uji statistik Anava Dua Jalur dan uji lanjut dengan uji Tukey berbantuan SPSS for windows. Hasil penelitian menujukan; (1) kemampuan menulis bahasa Inggris siswa yang mengikuti pembelajaran model kooperatif berbantuan asesmen portofolio lebih tinggi daripada siswa dengan pembelajaran konvensional (Fhitung= 4,249 > Ftabel(α0,05) =2,95 (2) terdapat interaksi antara model pembelajaran dengan kecemasan siswa (Fhitung = 4, 732 >Ftabel =2,95 (3) pada siswa dengan tingkat kecemasan tinggi, kemampuan menulis bahasa Inggris siswa dengan model kooperatif berbantuan asesmen portofolio lebih tinggi dibandingkan siswa yang mengikuti pembelajaran model konvensional Qhitung 7,91 > Qtabel=2,95 (4) pada kelompok siswa dengan kecemasan rendah, kemampuan menulis bahasa Inggris siswa dengan model kooperatif berbantuan asesmen portofolio lebih tinggi dibandingkan siswa yang mengikuti pembelajaran model konvensional (Qhitung = 5,28 > Qtabel =2,95Kata Kunci : asesmen portofolio, kecemasan siswa, kemampuan menulis bahasa Inggris, model pembelajaran kooperatif This research aims to investigate the effect of conventional learning model assisted by portfolio assessment towards student’s English writing skill compared to conventional learning model. It is also aimed to have the interaction effect of anxiety towards student’s English writing skill. This research was experimental research with 2x2 factorial design. The population was entire seventh grade junior high school students SMP Negeri 1 Payangan academic years 2013/2014 which were 287 students, and 84 students were assigned as research sample. Data in this research were collected using writing skill test and anxiety questionnaire, then analyzed using two-way Anova statistical analysis and continued with Tukey test. Research results show; (1) English writing skill of students who followed cooperative learning model assisted by portfolio assessment is higher than students who used conventional learning (Fobs= 4.249>Fcv =2.95); (2) there is interaction between learning model and student’s anxiety (Fobs=43.742>Fcv = 2.95); (3) on students with high anxiety level, English writing skill of students with cooperative model assisted by portfolio assessment is higher than students who followed conventional model (Qobs = 7.91 >Qcv = 2.95); (4) on group of students with low anxiety level, English writing skill of students with cooperative model assisted by portfolio assessment is higher than students who followed conventional model (Qobs = 5.28 >Qcv = 2.95). It can be concluded that cooperative learning model assisted by portfolio assessment and student’s anxiety have an effect on English writing skill of seventh grade junior high school students SMP Negeri 1 Payangan academic years 2013/2014keyword : conventional, cooperative learning model, English writting skill, portfolio assessment, student’s anxiety