Free volume characteristics in three samples of monodisperse polystyrene were investigated by positron annihilation technique over a temperature range from 300 to 380 K. The number-average molecular weight of the samples ranged from 5730 to 1,524,000. The observed lifetime spectra were resolved into three components, where the longest lifetime, tau(3), was associated with the pick-off annihilation of ortho-positronium (o-Ps) trapped by free volumes. The change of the temperature coefficient of tau(3) was observed at around 350 K, at which the value of tau(3) was a constant value of 2.3 ns for all specimens with different molecular weights. There was no discrete change of tau(3) in intensity, which is corresponding to the number of free volumes. The size of free volume at glass transition was evaluated to be 0.1 nm(3).