Ionic liquids (ILs) are considered as one of the potential heat transfer fluids (HTFs) for solar thermal applications where it comprised of base ILs and a small volume/weight percentage of nanoparticles. Dispersion stability of ILbased nanofluids is one of the challenges for make it as a viable HTFs. This paper presents the stability of ionic liquid based nanofluids for different sizes of nanoparticles and different concentrations by using visual inspection. The IL-based nanofluids contain 1-Butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₄mim][NTf₂]) and Al₂O₃ nanoparticle with 0.5 wt% and 1 wt% concentrations. Nanoparticles sizes are: 10nm, 30nm, 60nm, and 90nm. The results demonstrated that the ionic liquid's stability generally decreases as nanoparticle size and concentration increases. Increased mixing time also improved samples' stability but only for samples at a 0.5 wt% concentration. The 10 nm nanoparticles, the smallest observed, proved to be the most stable over the longest period when mixed for 90 minutes and concentrated at 0.5%. The most consistent trend was observed in the samples containing 30 nm nanoparticles, which were the most unstable across all mixing times and concentrations.