AbstractThis research aimed at knowing whether there was or not an effect and increase of values of Community of Inquiry (CoI) based Blended Learning model toward student learning achievement. It was Quasi Experimental research—a research that used an Experimental group and a Control group determined the difference of two variables. The samples of this research were 2 groups—the tenth-grade students of Natural Science 4 (the Experimental group) that were 36 students taught by using Community of Inquiry based Blended Learning model and the students of Natural Science 5 (the Control group) that were 34 students taught by using Conventional learning. The techniques of collecting the data were written pretest and posttest. The collected data were analyzed by using Independent Sample t test and to see the effect of using the coefficient of determination. The final data processing results obtained tcount of 1.778 while the value of ttable at a significant level of 5% = 1.684 then H0 is rejected and Ha is accepted. Means there is an influence of blended learning model based on community of inquiry (CoI) with conventional learning models on student learning outcomes. To determine the magnitude of the effect of the treatment used the coefficient of determination formula and the results obtained by 0.04, so the coefficient of influence (Kp) obtained is equal to 4%.Keywords : Blended Learning, Community of Inquiry, Learning Achievement, Atomic Structure and Periodic System of the ElementsAbstrakPenelitian ini bertujuan untuk mengetahui ada atau tidaknya pengaruh dan peningkatan nilai model Blended Learning berbasis Community of Inquiry (CoI) terhadap prestasi belajar siswa. Merupakan penelitian kuasi Eksperimental—penelitian yang menggunakan kelompok Eksperimen dan kelompok Kontrol menentukan perbedaan dua variabel. Sampel penelitian ini adalah 2 kelompok yaitu siswa kelas X IPA 4 (kelompok eksperimen) yang berjumlah 36 siswa yang diajar dengan model Blended Learning berbasis Community of Inquiry dan siswa IPA 5 (kelompok kontrol) yang sebanyak 34 siswa diajar dengan menggunakan pembelajaran konvensional. Teknik pengumpulan datanya adalah pretest dan posttest tertulis. Data yang terkumpul dianalisis dengan menggunakan Independent Sample t-test dan untuk melihat pengaruhnya menggunakan koefisien determinasi. Hasil pengolahan data akhir diperoleh t-hitung sebesar 1,778 sedangkan nilai t-tabel pada taraf signifikan 5% = 1,684 maka H0 ditolak dan Ha diterima. Berarti ada pengaruh model pembelajaran blended learning berbasis community of inquiry (CoI) dengan model pembelajaran konvensional terhadap hasil.Kata Kunci: Blended Learning, Komunitas Inkuiri, Prestasi Belajar, Struktur Atom dan Sistem Periodik Unsur