In the present investigation, the concrete decontamination and decommissioning (D&D) process with a finite radiating microwave source is numerically simulated. Power dissipation within a concrete slab is evaluated by solving Maxwell's equation in a near field after the waveguide. Using the finite difference method, the transient temperature field induced by the microwave power dissipation is solved. The effects of microwave frequency, applicator size, and microwave operating mode on temperature distribution are investigated. For the four commonly used industrial microwave frequencies of 0.896, 2.45, 10.6 and 18.0 GHz, it has been found that higher frequency results in high power dissipation and thus in a higher temperature field. Similarly, a larger applicator leads to a higher temperature distribution. When the operating mode changes from TE10 to TE20 and TE30, the amplitude of temperature decreases in order, and temperature patterns become more and more flat.