The plastic deformation behavior of commercially pure Ti single crystals has been investigated by uniaxial micropillar compression tests as a function of crystal orientation and specimen size at room temperature. {1011} (first-order) pyramidal c+a slip and prism a slip are activated in micropillar specimens with the [0001] and [2110] orientations, respectively. {1011} pyramidal c+a slip has never been observed to operate as a major deformation mode in compression tests of ‘bulk’ single crystals at room temperature, in which {1122}<1123> twinning is usually observed. The CRSS values for {1011} pyramidal c+a slip and prism a slip increase with the decrease in the specimen size, following an inverse power-law relationship with a power-law exponent of about 0.06 and 0.59, respectively. The extrapolation of the inverse power-law relationship up to the ‘bulk’ specimen size estimated from the CRSS values of prism a slip gives the ‘bulk’ CRSS value for {1011} pyramidal c+a slip to be 580-635 MPa, which is by far higher than those for any other deformation modes operative at room temperature.