β-FeSi2 is important material for silicon based optoelectronics. Direct and indirect bands are located close each other and due to strain effects in some epitaxial configuration direct band can be lower than indirect one. Therefore conditions of preparation are very important factor determining the final properties of material. The aim of this paper is to study morphology and optical properties of β-FeSi2 grown by reactive deposition epitaxy (RDE) on Si substrate and influence of annealing conditions. Thin 6 nm iron film has been deposited by e-gun on p-type Si (100) at 600°C. Obtained metal/semiconductor structures (MSS) have been immediately annealed at 600°C in the same vacuum chamber for different time: 10 min (sample #1), 20 min (#2) and 30 min (#3) to produce iron silicide. AFM study shows stand alone nanoparticles as well as agglomerates of them. Particles size gradually increases with annealing time. Freshly evaporated Fe film is also granulated with particle size around 6 nm which coincides with film thickness. Particle size distribution curve for MSS has 2 maxima. First one is for single particles and second one is for groups of particles. They are ~20; ~30; and ~40 nm for single particles and samples #1; #2; and #3 respectively. For agglomerates of particles we have ~130; ~150 and ~80 nm sizes. Variable angle ellipsometry and reflectance of p-polarized light have been used to study optical constants and thickness of the film which includes FeSi2 nanodots. Single layer optical model with effective medium approximation was applied. The surface film thicknesses obtained from such measurements are from 6 to 9 nm. Longer annealing time produces a thicker silicide film due to crystal growth and surface roughening. Samples annealed for 10 and 20 min are similar with almost same surface film thickness while for the sample with 30 min of annealing we observe lower n and k values with increased film thickness. There is a little difference between optical constants of Fe and surface film of samples #1,#2. This difference is larger for the sample #3. The AFM and optical data shows that there is a free iron on silicon surface for all annealing time. RDE at these conditions does not form continues β-FeSi2 film but nanoparticles instead.