GaAsN epitaxial thin films were grown on GaAs (001) substrate by chemical beam epitaxy with dimethylhydrazine ((CH3)2N 2H2). In the dependence of growth temperature on the N concentration, there were three distinct regions in which the dependence was different. At the growth temperature of 420°C, the N concentration increased with increasing surface step density that corresponds to misorientation angle of GaAs (001) substrate. When the samples grown on 2 and 10° off substrates were compared, the root-mean-squares of surface roughness decreased from 1.1 nm to 0.3nm, despite of the increase of N concentration. In addition, the reduction of the impurity concentrations (hydrogen and carbon) in the sample grown on the 10° off substrate was also observed. These results showed the surface morphology of the GaAsN thin films is affected by the step density, and the possibility to obtain (In)GaAsN thin film that has high N and low impurity concentration with keeping crystal quality.