Heterojunction structures were fabricated, which consisted of a double-walled carbon nanotube (DWCNT) thin film coated either on an n-type silicon wafer or an n-type silicon nanowires (SiNW) array with varied lengths. Current–voltage characteristics measured under laser (532 nm) irradiation showed that the photoresponse [(light current  dark current)/dark current] of the heterojunctions dramatically depends on the length of SiNWs. Increase in the length of SiNWs led first to increase and then to decrease in the photoresponse of DWCNT/SiNW heterojunction. The heterojunction with a SiNW length of ca. 600 nm has the highest the photoresponse value of 10.72.