This paper introduces the design, implementation and measurements of a GaN-based SSPA(Solid-State Power Amplifier) which is applicable to the radar system with more than 1.6 kW output in S-Band. SSPA consists of HPA(High Power Amplifier) module, DA(Drive Amplifier) module, digital control module and power supply module. HPA is composed of parallel configuration of 250 W high power amplifier adopting GaN HEMT. The HPA is also designed with the 8-way divider/coupler adopting LTCC. DA accommodates an ALC(Automatic Level Control) function. Error detection sensor in digital control module improves the operator"s convenience by displaying operation and error status. In addition, the SSPA is verified as a reliable military component by conducting MIL-STD level environmental test.