emissivity is a dimensionless ratio (i supppose it might be better called relative emissivity)
see http://en.wikipedia.org/wiki/Emissivity" …
The emissivity of a material (usually written ε or e) is the relative ability of it's [spelling!] surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature. A true black body would have an ε = 1 while any real object would have ε < 1. Emissivity is a dimensionless quantity.
Thanks for the reply tiny-tim. I know what is emissivity and i want to prove that emissivity is dimensionless. I know that is because is a ratio but i want to prove it through equations(dimensional analysis-using mass,length,time).