This paper suggest a monitoring method for the pulsed laser spot welding of the thin metal sheet using a point IR sensor. A new criterion was introduced and the experimental results guaranteed the effeciency. The ideal radiation feature was derived from the mathematical model and was simulated. The radiation feature is rubust to withstand the change of measuring condition and can be used to detect the absorbed laser energy. In an experiment, the radiation feature was examined for the different laser energy. The pulse width and the laser power was variated and the radiation feature was examined. In the other experiment, the relationship between the weld strength and radiation feature was examined. Artificial Neural Network was employed to find out the relationship. The correlation coefficient between the real strength and the estimated strength is high as 0.94 and the mean square error is low as 0.64kgf learned parts. Another group of the welds was used to appraise the learning efficiency. The correlation coeffecient between the measured and the estimated weld strength is high as 0.91.