Tujuan penelitian pengembangan ini untuk menghasilkan produk yang layak meliputi: kevalidan, kepraktisan, dan keefektifan media Let’s Do It berbantuan website Lumio By Smart kelas IV Sekolah Dasar. Dilatarbelakangi oleh adanya kemampuan kosakata Bahasa Inggris peserta didik yang kurang optimal pada Unit 1: What Are You Doing . Subjek penelitian ini adalah peserta didik kelas IV-D dan IV-B SDN Wiyung I/453 Surabaya. Penelitian ini menggunakan jenis penelitian Research and Development (R&D) dengan model pengembangan Analys, Design, Development, Implementasi, and Evaluation (ADDIE). Teknik pengumpulan data menggunakan instrumen validasi ahli materi dan media, lembar angket guru, lembar kepraktisan, lembar keefektifan, dan pretest-posttest. Hasil uji validasi materi sangat layak dengan persentase 96%, sedangkan hasil uji validasi media juga sangat layak dengan persentase 92%. Media dikatakan sangat praktis digunakan dengan hasil perhitungan persentase 100%. Rata-rata skor tes adalah 70 dan 62,5 (pretest) serta 95 dan 92,25 (posttest). Penilaian ketuntasan (PK) mendapat persentase sebesar 100% pada posttest, yang artinya peserta didik memahami materi dan media secara optimal. N-gain score 0.8 > 0,7 dan N-Gain persen nya 88 > 75 untuk skala kecil. N-gain score 0,76 > 0,7 dan N-Gain persen 76.6 > 75 untuk skala besar. Dengan demikian dapat disimpulkan bahwa media Let’s Do It berbantuan website Lumio By Smart materi Unit 1: What Are You Doing dinyatakan memenuhi kriteria valid, sangat praktis, dan efektif serta dapat membantu peserta didik kelas IV dalam mengoptimalkan kosakata Bahasa Inggris.

Kata Kunci: Pengembangan, Lumio By Smart, Kosakata.

The purpose of this development research is to produce decent product which includes: validity, practicality, and effectiveness of Let's Do It media assisted by Lumio By Smart website for grade IV Elementary School. It is motivated by the presence of less than optimal English vocabulary skills of students in Unit 1: What Are You Doing. The subjects of this study were students of class IV-D and IV-B SDN Wiyung I/453 Surabaya. This research uses the Research and Development (R&D) research type with the Analyze, Design, Development, Implementation, and Evaluation (ADDIE) development model. The data collection technique used material and media expert validation instruments, teacher questionnaire sheets, practicality sheets, effectiveness sheets, and pretest-posttest. The results of the material validation test were very feasible with a percentage of 96%, while the results of the media validation test were also very feasible with a percentage of 92%. The media is said to be very practical to use with a percentage calculation result of 100%. The average test scores were 70 and 62.5 (pretest) and 95 and 92.25 (posttest). The assessment of completeness (PK) received a percentage of 100% on the posttest, which means that students understand the material and media optimally. N-gain score 0.8 > 0.7 and N-Gain percent 88 > 75 for small scale. N-gain score 0.76 > 0.7 and N-Gain percent 76.6 > 75 for large scale. Thus it can be concluded that the Let's Do It media assisted by the Lumio By Smart website Unit 1: What Are You Doing material is declared to meet the criteria of valid, very practical, and effect.

Keywords: Development, Lumio By Smart, Vocabulary.