Curcumin is a polyphenol compound contained in turmeric rhizome plants, and determines its pharmacological activity. Modification of curcumin structure has been explored to increase the lipophilicity of curcumin. The increase of lipophilicity is expected to enhance its bioactivity. This study aims to modify hydroxyl groups of curcumin by O-ethylation with diethyl carbonate that is environmentally friendly. Curcumin was modified into diethyl curcumin with a K2CO3 base catalyst under reflux conditions. TBAB (tetrabutylammonium bromide), as a Phase Transfer Catalyst (PTC), is added to this reaction system and has been shown to improve the synthesis results. The product has obtained percent yield of 89 % and characterized as diethyl curcumin. Curcumin and diethyl curcumin have shown an antibacterial activity against the growth of Gram-negative Escherichia coli bacteria, but did not inhibit the growth of Gram-positive Staphylococcus aureus bacteria.