A semiconductor device includes a plurality of first memory strings each first memory string having a channel with a first length and a plurality of second memory strings each second memory string having a channel with a second length shorter than the first length. A method of operating the semiconductor device includes: performing a first read operation on the first read unit, wherein the first read unit includes the first memory cells sharing the same first word line among first memory cells included in the plurality of the first memory strings; and performing a second read operation on the second read unit, wherein the second read unit includes the second memory cells sharing the same second word line among second memory cells included in the plurality of the second memory strings.