This paper describes the design and performance of a serial scan thermal imaging system with emphasis on the design of scanners. The sensor with a single-element HgCdTe detector employs a oscillating mirror and a truncated hexagonal pyramid-type mirror as vertical and horizontal scanners, respectively. The size of the scanners was optimized to be as compact as possible by disposing each axis of scanners at the real and horizontally scanned position of exit pupil of the afocal system respectively. This condition also reduces the amount of migration of the scanned parallel beam, which has made easy to design a diffraction limited focusing lens. The image distortion and timing error of horizontal sweep caused by the rotating scanner are compensated by signal processing techniques which have been proven to give satisfactory system performances. The system has the following parameters: field of view; 40 x 53 mrad, angular resolution; 0.4 mrad, diameter of afocal telescope; 100 mm, frame rate; 20 sec -1, and 121 TV lines with a 2 to 1 interlace. The results of system performance test have shown that NETD is 0.6°C and MRTD is 0.6°C at 1 cy/mrad which are close to the design values.