A vertical transistor includes a source-channel-drain structure, a gate and a gate dielectric layer. The source-channel-drain structure includes a source, a drain over the source and a channel between the source and the drain. The gate surrounds a portion of the channel. The gate is configured to provide compressive strain substantially along an extending direction of the channel when the vertical transistor is an n-channel vertical transistor, or the gate is configured to provide tensile strain substantially along the extending direction of the channel when the vertical transistor is a p-channel vertical transistor. In some embodiments, the vertical transistor further includes an ILD configured to provide tensile strain substantially along an extending direction of the channel when the vertical transistor is an n-channel vertical transistor, or configured to provide compressive strain substantially along an extending direction of the channel when the vertical transistor is a p-channel vertical transistor.