The effect average alumina nanoparticle sizes on the long term dispersion stability of biodiesel based nanofluids was investigated. Alumina nanoparticles having two different average sizes (~13nm and ~28nm) were dispersed in the Jatropha biodiesel as the base fluids. The effect of alumina (Al2O3) nanoparticles sizes on the stability of nanofluids was investigated to achieve more stable nanoparticles dispersed nanofluids having longer duration for potential use in alternative fuel energy applications. Different volume fractions (VF) such as 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% nanofluids were prepared by using two different sizes of alumina nanoparticles (~13nm and ~28nm) by using the surfactants Span 80 and Tween 80 in the ratio of 1:1. The results revealed that the nanofluids having the smaller average sizes alumina nanoparticles and 0.1% volume fraction were stable for more than one year as compared to the larger (two times) size nanoparticles having same 0.1% volume fraction. Such long term stable biodiesel based nanofluids can be used as the alternative fuel energy for future automobiles and transportation sectors due to the fuel properties of such nanoparticles dispersed nanofluids retaining the commercial diesel properties.