The crystallization of hydrogenated nanostructured silicon (ns-Si:H) films deposited from Ar-silane mixture in a low-pressure pulsed radio-frequency glow discharge has been studied in relation with their structural and morphological properties. Different techniques' of characterization converge to the fact that both the porosity and the surface roughness of the film increase with the plasma duration (T-on) used for the deposition. The correlation between the film structure and the crystallization threshold has been investigated. The modifications of the bulk structure of the film with T-on partly explain the decrease of the crystallization threshold (E-cryst) The role of the surface roughness in the lowering of the crystallization threshold is emphasized. Its contribution is interpreted by the enhancement of the electromagnetic field at the ns-Si:H him surface.