Nanowire-like, condyloid-like and flakes of Si-nanostructures were synthesized by thermal evaporation under different mass transport conditions by changing the ambient pressure. The structural analysis shows that a higher mass transport rate is not favourite for the formation of fine single crystalline nanowires when the substrate placed closely to the thermal vapour source. The higher mass transport rate can induce a lower Si partial pressure near the source and hence results in a lower supersaturation near the substrate. Experimental results reveal that the formation of Si-nanowires is not controlled by mass transport but by surface process. The driving force on the surface is the key factor for the formation of well-crystallized nanowires.