A semiconductor device is provided, which includes a first and second multichannel active patterns spaced apart from one another and extending in a first direction. The semiconductor device also includes first and second gate structures on the first and second multichannel active patterns, extending in a second direction and including first and second gate insulating films, respectively. Sidewalls of the first multichannel active pattern include first portions in contact with the first gate insulating film, second portions not in contact with the first gate insulating film, third portions in contact with the second gate insulating film, and fourth portions not in contact with the second gate insulating film. Additionally, a height of the first portions of the first multichannel active pattern is greater than a height of the third portions of the first multichannel active pattern.