The present investigation aimed to study the effect of particle size of solid lipid nanoparticles (SLNs) on oral bioavailability of darunavir. High pressure homogenisation technique was used to prepare SLNs. Three different sized SLNs loaded with darunavir were developed with mean particle sizes of around 100 nm, 200 nm and 500 nm, respectively. The in vivo pharmacokinetics in rats showed a significant increase in oral bioavailability of darunavir from all the three formulations in comparison to plain drug suspension and reference tablet. The results revealed insignificant difference between SLNs of 100 and 200 nm and these had significantly higher bioavailability in comparison to SLNs of 500 nm. However, more number of homogenisation cycles is required for obtaining 100 nm and thus we selected 200 nm as an optimum size for oral bioavailability enhancement of darunavir. The optimised SLN formulation was stable for a period of 6 months at 25 +/- 2 degrees C/60 +/- 5% relative humidity (RH).