During the last decades, photocathode rf gun has been proven to be
successful in generating the high brightness electron beam
(~1nC,~1πmmmrad,~1 ps) which is required by the ILC,
XFEL, Thomson scattering x-ray source, etc. A photocathode rf gun system
is built to develop electron source for the Thomson scattering x-ray
source at Accelerator Laboratory of Tsinghua University. The system
consists of a BNL/ATF-type 1.6 cell S-band rf cavity, a solenoid for
emittance compensation, a laser system and some simple equipments for
beam diagnosis. The first beam measurements of the photocathode rf gun,
including the dark current, transverse beam profile, charge and quantum
efficiency, are reported.