A numerical model has been developed for thorough investigation of the radiation and air-preheating effect on the sooting behavior of a diffusion flame. An explicit finite difference scheme has been adopted for the solution of different conservation equations for reacting flows along with the soot conservation equations. A variable size adaptive grid system has been considered using hyperbolic distribution to capture the sharp gradients of the field variables. Temperature distribution pattern does not change when radiation effect is considered, but peak temperature is lowered due to radiative heat loss. On the other hand, air-preheating not only increases the peak temperature, but also changes the distribution pattern significantly. Radiation decreases the soot volume fraction and soot number density to a large extent both for non-preheated and preheated case. Soot diameter is not much affected due to radiation, but it increases with preheating of air.