We performed growth of body-centered-cubic Nb thin films using an electron cyclotron resonance (ECR) technique on {100} Si substrates. We deposited Nb films using both Ar and Xe plasmas. Reflection high-energy electron diffraction and atomic force microscopy analyses have been performed in order to observe the surface topography. We found that the surface roughness of films deposited by ECR is 4-5 times smoother than films deposited by magnetron sputtering systems. We obtained this result even at room temperature deposition conditions.