In this study, the in situ transesterification of jatropha seed using hydrodynamic cavitation for the production of biodiesel was studied. Experiments were carried out under the following conditions: 800 mL of methanol volume, 50oC of reaction temperature, 2 g of potassium hidroxide catalyst, less than 3% of moisture content in jatropha seed and 0,355- 1,18 mm of particle size. From experiment without co-solvent addition, the highest biodiesel yield of 35% obtained at condition: time of process of 120 minute and ratio of methanol to jatropha seed of 16 (mL/g) . The use of co-solvent of hexane provided a higher yield of biodiesel compared to the system without co-solvents. The highest biodiesel yield of 60% were obtained under the addition of 95 mL of hexane volume. In addition, the use hydrodynamic cavitation provided a higher yield of biodiesel compared to the system using mechanical stirring. The biodiesel produced in the experiment was analyzed by gas chromatography-mass spectrometry (GC-MS), which showed that methyl oleate was the highest compound in biodiesel. Keywords: In situ transesterification, hydrodynamic cavitation, biodiesel, jatropha curcas seed, co-solvent   ABSTRAK Penelitian ini bertujuan untuk mempelajari proses produksi biodiesel dari biji jarak pagar melalui proses transesterifikasi in situ menggunakan kavitasi hidrodinamik. Transesterifikasi in situ dilakukan pada kondisi: volume metanol 800 mL, temperatur proses 50oC, katalis kalium hidroksida 2 g, kandungan air biji jarak kurang dari 3% dan ukuran partikel biji jarak 0,355-1,18 mm. Hasil penelitian menunjukkan rendemen biodiesel tertinggi sebesar 35% yang dihasilkan dari proses transesterifikasi in situ biji jarak pagar menggunakan kavitasi hidrodinamik tanpa penambahan co-solvent diperoleh pada penggunaan waktu proses 120 menit dan rasio volume metanol terhadap berat biji jarak 16 (mL/g). Penggunaan co-solvent heksana terbukti dapat meningkatkan rendemen biodiesel yang dihasilkan dimana pada penambahan volume heksana sebanyak 95 mL diperoleh rendemen biodiesel sebesar 60%. Proses transesterifikasi in situ biji jarak menggunakan kavitasi hidrodinamik menghasilkan rendemen biodiesel yang lebih besar dibandingkan proses menggunakan pengaduk mekanik. Hasil analisis gas chromatography-mass spectrometry (GC-MS) menunjukkan komponen terbesar dalam produk biodiesel adalah metil oleat. Kata kunci: Transesterifikasi in situ, kavitasi hidrodinamik, biodiesel, biji jarak pagar, co-solvent