Monitoring seawater temperature is important to understanding evolving ocean processes. To monitor internal waves or ocean mixing, a large number of temperature loggers are typically mounted on subsurface mooring systems to obtain high-resolution temperature data at different water depths. In this study, we redesigned and evaluated a compact, low-cost, self-contained, high-resolution and high-accuracy ocean temperature logger, the TC-1121. The newly designed TC-1121 logger is smaller and more robust, and its sampling interval can be automatically changed by indicated events. The loggers initial accuracy is +/- 0.002 degrees C, and its effective resolution is about 0.0001 degrees C. The drift error of all six TC-1121 temperature loggers during a 450-day experiment was less than +/- 0.002 degrees C. They are being widely used in many mooring systems to study internal wave and ocean mixing. The loggers fundamental design, noise analysis, calibration, and drift test are discussed. A long-term sea trial in the South China Sea was completed successfully and demonstrated the effectiveness of the logger.