Critical dimension uniformity is one of the key parameters to define the performance of electron beam lithography as well as the other lithography technologies. The characteristics of poly-butene-sulfone (PBS), a positive-type electron resist, have been investigated from the view-point of critical dimension control, using two types of electron beam lithography systems which use Gaussian spot and variable-shaped beams. It has been elucidated that the sensitivity of the PBS has a strong relationship with humidity during development. When the humidity changes from 30% to 32%, the sensitivity increases corresponding to the increase of 1 µ C/cm2 dosage. A very good critical dimension uniformity of less than 0.04 µ m has been attained using a variable-shaped beam at 30% humidity. The spot beam can also obtain a good pattern width uniformity at 32% humidity and 4.5 µ C/cm2 dosage. It is expected to correct the proximity effect in PBS by selecting the electron dosage related to humidity.