Optical bistability(OB) in semiconductors is of increasing interest in both basic aod applied lesearch. Much is known about optical nonlinearity and OB

and several kinds of optical bistability have been observed in CdS crystals and films. This paper leports the optical properly and bistability in CdS epilayers grown on CaF

2

tiansparent substrates by vapour phase epitaxy(VPE) for getting large area

uniform and high quality samples. The source material v as the high quality CdS single crystal and the superpurity H

2

gas yas used as carrier gas. The freshly cleaved CaF

2

substrate was put on the deposit range

then raised the lemperature of source material to 700-800℃ and that of substrate to 500-600℃ in H

2

flow. The emission and transimission spectra were measured by 44W grating monochro-meter and recorded by cooled pholomultiplier. The 5145? line of Ar

+

laser chopped into 6ms light pulses was focused to the radius of about 50um for OB measurement at room temperature.