An On Die Thermal Sensor (ODTS) of a semiconductor memory device includes: a temperature detector for detecting an internal temperature of the semiconductor memory device to generate a temperature voltage corresponding to the detected internal temperature; a tracking ADC for outputting a digital code by comparing the temperature voltage with a tracking voltage and performing a counting operation to the result of comparison; and an operation controller for controlling operations of the temperature detector and the analog-to-digital converter, wherein the tracking ADC performs the counting operation using a first tracking scheme having a relatively large unit variation width of the digital code value during an initial tracking period and a second tracking scheme having a relatively small unit variation width of the digital code value after the initial tracking period.