Numerical simulation of heat transfer on a 6U CubeSat is performed. The spacecraft's primary mission is a space environment demonstration of a NASA Innovative Advanced Concepts (NIAC) project for the measurement of asteroid composition using a high-power laser. The laser works on cycles of 10 seconds on and 10 minutes off. Approximately 200 W is dissipated as heat when the laser is on. Attitude control mechanisms maintain the laser pointing towards the target, and also keep the spacecraft between the sun and the target. Simulations are performed using ANSYS CFX; boundary conditions are inserted as User Defined Functions. Mechanical design includes a network of heat pipes and passive radiative surfaces for heat distribution and expulsion. Four cases are addressed, all with orbit inclination of 90° and 650 km altitude. Two simulations have ascending node equal to 90°, named as hot cases. The remaining two cases have ascending node equal to 0, referred to as cold cases. For the hot case and laser turned off, the maximum and minimum temperatures obtained are 344 K and 282 K, respectively. Temperatures of 352 K and 287 K are found for this same orbit, but with the laser on. For the cold case and laser always off, extreme onboard temperatures are 334 K and 263 K, and with laser working, extremes are 342 K and 268 K. In all simulations the highest temperatures are on surfaces facing the sun while the lowest are on the opposite side. For the operating mode of the laser, its results for range of temperature on orbit are outside the suggested from manufacturer.