The transient microbending loss in a carbon-coated optical fiber, subjected to time-dependent thermal loading, is analyzed. First, the temperature distributions in the fiber and coating, which are varying with time, are derived. Then, the thermally induced, time-dependent compressive lateral pressure at the interface of the fiber and the coating is obtained. Finally, the transient microbending loss of the carbon-coated fiber is presented. It has been found that the transient microbending loss in the carbon-coated optical fiber increases with increasing Young's modulus, Poisson's ratio, and thermal expansion coefficient of the carbon coating. Therefore, in order to minimize the transient microbending loss of the carbon-coated optical fiber, the Young's modulus, Poisson's ratio, and thermal expansion coefficient of the carbon coating should be decreased. Nevertheless, there exists an optimum value for the thickness of the carbon coating to reduce the transient microbending loss and sustain the mechanical force at the same time.