The purpose of this paper is to evaluate the performance of the calculation algorithm of the TDLAS (tunable diode laser absorption spectroscopy). Previous calculation algorithms used for reconstructing the temperature and concentration fields in the TDLAS utilize the peak intensity information of the representative waves at 1388.135nm and 1388.326nm for the case of H2O vapor. In this paper, a new method in which correlation coefficients between the wave patterns obtained by experiments and those obtained by theoretically calculated values are used for reconstructing the temperature and concentration fields is proposed. That is, the light intensities obtained from experiments and were compared with those obtained from the theory were matched to get final matched patterns. For reconstructing the temperature and concentration fields, the MART (Multiplicative Algebraic Reconstruction Technique) which is widely used for CT-based reconstruction algorithms was used. Calculation errors are relatively larger at higher at higher temperature zone than lower temperature zones. It was verified that larger errors at high temperatures was due to a very weak intensity change.