For a droplet burning in quiescent air and in a high temperature environment, conduction and radiation are two major heat transfer modes. Soot formation has been observed in many experiments on single-droplet combustion of different fuels. As a result, radiation heat transfer should play a dominant role in the burning process. In addition, most liquid fuels are semi-transparent materials. The collocation method is applied to solve the radiative heat transfer equation in regard to the droplet interior. The results, including different cases of droplet diameter square, ignition delay time, with and without considering radiation heat transfer, are compared with the experimental data. The prediction of the diameter square when radiation heat transfer is considered agrees more with the experimental data than the case without considering radiation heat transfer.