The present investigation was aimed to study the antiproliferative action of Tamoxifen citrate (TMX), a non steroidal antiestrogen, on a human T-cell leukemia cell line, Jurkat, as free drug and TMX loaded guar gum nanoparticles. For this we developed a new formulation containing chemically cross-linked guar gum nanoparticles (GG NPs) loaded with Tamoxifen citrate (TMX). Single step (oil in water) emulsion and in-situ polymer crosslinking technique was employed to prepare spherical and smooth surfaced nanoparticles in the size range of 200-300nm. Nanoparticle size and shape was confirmed from observation in transmission electron microscope (TEM) analysis. Cytotoxicity on Jurkat (human T-cell leukemia) cell lines as determined by cell growth inhibition after 48 hrs of incubation indicated that Tamoxifen citrate loaded guar gum nanoparticles were as efficient as the free drug when applied to the cancer cells. However, the crosslinked guar gum nanoparticles loaded with Tamoxifen citrate exhibited sustained release of the drug and delayed apoptosis over a long period of time making it suitable for cancer treatment.