Human is homeotherm that maintains the body temperature within a narrow range despite the large changes in environmental temperature. This successive control is important for optimal enzymatic reaction in the body and at the same time provides an ideal environment for cells and organs to function. We, human, is also an endotherm that generates our own internal heat, in response to external and internal environments. The core body temperature or the internal temperature in the organ for a human is 37°C or 98°F and ranged with the standard deviation of 0.2°C. The different temperature obtained from different part of the body. Authors like to draw the reader to understand the physiology of human thermoregulation. This book will elaborate on the mechanism of maintaining the body temperature in response to the external and internal environment.