© 2016 IEEE. An in-house 3D Finite Element (FE) Monte Carlo (MC) toolbox is used to study the effects of uniaxial tensile strain in nanoscale Si n-channel SOI FinFETs with two channel orientations ((100) and (110)). We simulate a FinFET with a rectangular-like cross-section (4.5 nm × 11 nm) and a gate length of 8.1 nm with EOT=0.55 nm and study the effects of two types of tensile strain: uniaxial (100) and uniaxial (110) with strain strengths of 0.5%, 0.7% and 1.0%. To show how quantum confinement can degrade the effectiveness of strain engineering, we compare the results with a bigger device with a rectangular-like cross section (12 nm × 30 nm) and a gate length of 25 nm with EOT=1.12 nm. It is found that applying the uniaxial (100) strain increases more the on-current than the uniaxial (110) strain. Moreover, with increasing the strain strength, the quantum confinement induced pre-existing valley splitting starts to weaken the strain effect especially in the (110) channel orientation.