In this research, Sm 2 O 3 nanoparticles (NPs) have been prepared via Caesalpinia pulcherrima leaf extract (CPE), whichh contains secondary metabolites contained as a weak base source and a capping agent. The nanoparticles were confirmed by UV-Vis DRS, FT-IR spectroscopy, XRD, PSA and TEM. FT-IR spectrum showed that CPE contains the secondary metabolites which play an essential role in the preparation of Sm 2 O 3 NPs. UV-Vis DRS result showed that the bandgap value of Sm 2 O 3 NPs was approximately 4.76 eV which works in the UV region. Based on the XRD analysis, Sm 2 O 3 NPs have a structure of cubic. PSA characterization showed that the average particle size distribution of Sm 2 O 3 was around 84.47 nm. TEM image confirmed that Sm 2 O 3 was in nanoscale with the particle size of 73.27 nm. Finally, Sm 2 O 3 NPs have been tested for the photodegradation of malachite green (MG) under UV light illumination. The degradation percentage of MG using Sm 2 O 3 NPs photocatalyst was 80.14% for 2 h.