The spectral emission of the laser induced plasma is investigated during the welding process of steel. In this study emission spectroscopy in the visible spectral range was used. A method is presented for determining the electron temperature, the electron density as well as the density of the neutral atoms in the welding plasma. In these metallic studies the relative line intensity technique was used, in which the radiation from a pair of spectral lines is compared as a function of electron temperature. The technique eliminates the need for a calibration source and can be used easily in the plasma monitoring applications. Some examples of the estimations of the plasma temperature, the electron and the neutral atom density are demonstrated. Time and space resolved measurements of the keyhole emission are presented. The application of the spectral diagnostic during the keyhole formation at the beginning of the metal sheet as well as at the end of the welding process at the end of the sheet is shown. The results help to explain the plasma absorption inside the keyhole. Some aspects of the spectral diagnostic as a basic method for a welding control are discussed.