Salah satu pemanfaatan kembali minyak jelantah atau Used Cooking Oil (UCO) adalah diolah menjadi biodiesel Used Cooking Oil Methyl Ester (UCOME). Free Fatty Acid (FFA) dalam UCO diolah melalui reaksi transesterifikasi menjadi metil ester. Penelitian ini bertujuan untuk mempelajari pengaruh kadar FFA UCO dan massa katalis terhadap karakteristik dan yield UCOME yang dihasilkan. Variasi kadar FFA UCO yang digunakan yaitu 1,493%; 1,536%; 2,56%; dan 5,504%. Reaksi transesterifikasi dilakukan pada temperatur 60 – 65 ℃, pengadukan 350 rpm, serta rasio mol UCO dan metanol (1:6) dengan variasi massa katalis KOH yaitu 0,5%, 1,5%, dan 2,5% (b/b) UCO. Parameter uji karakteristik UCOME meliputi densitas, viskositas, API Gravity, dan Higher Heating Value (HHV). Analisis komponen kimia dilakukan menggunakan alat Gas Chromatography-Mass Spectrometry (GC-MS). Yield tertinggi sebesar 96,59% diperoleh pada hasil transesterifikasi sampel dengan kadar FFA 1,493%. Massa katalis KOH yang optimal adalah 1,5% (b/b UCO). Hasil GC-MS produk dengan kadar FFA awal <5% didominasi oleh metil ester rantai C11-C19. Karakteristik seluruh produk dengan kadar FFA awal <5% memenuhi standar biodiesel menurut SNI.The Effect of Free Fatty Acid Content in Used Cooking Oil (UCO) and Catalyst Mass in Transesterification Process on Used Cooking Oil Methyl Ester (UCOME)’s Characteristics and Yield. One way to take advantage of Used Cooking Oil (UCO) is by recycling it into Used Cooking Oil Methyl Ester (UCOME) biodiesel. The free fatty acids (FFA) in UCO are processed through a transesterification reaction into methyl esters. This study aims to review the effects of FFA content in UCO and the catalyst mass used on the characteristics and yield of UCOME produced. The variations in FFA content in UCO are 1.493%, 1.536%, 2.56%, and 5.504%. The transesterification reaction was carried out at a condition of 60 – 65 ℃, with stirring at 350 rpm, and mole ratio between UCO and methanol (1:6) with variations in the amount of KOH catalyst at 0.5%, 1.5%, and 2.5% w/w UCO. The UCOME characteristic test parameters include density, viscosity, API gravity, and Higher Heating Value (HHV). The highest yield of 96.59% was obtained from the transesterification of the sample with an FFA content of 1.493%. The optimal amount of KOH catalyst is 1.5% w/w UCO. The GC-MS results of products with initial FFA content <5% are dominated by C11-C19 methyl esters. The characteristics of all products with initial FFA content <5% fulfill the biodiesel standards according to SNI.