Abstract: The influence of the three parameters,namely the beam size of Gaussian source,the source size and the detector size,on the three-dimensional optical transfer function(3-D OTF) of an incident light fluorescent confocal microscope is investigated,and the 3-D OTF of such a set-up is derived.The calculated results show that the beam size has an effect on choosing both the source size and the detector size by means of a gaussian beam with the smaller beam size,both the source size and the detector size can be increased,and better optical resolution and higher signal-noise-ratio can be reached,compared with those by means of a parallel beam.