Laser welding is a modern method of fusion welding. Due to high welding speeds is necessary to use the machinery welding. This leads to the need for the detection and eventually the elimination of weld defects. Penetration laser welding is not accessible to direct observation and therefore can be monitored only indirectly. A new of possibility is the study of plasma radiation forming over the keyhole using autocorrelation analysis. The intensity of plasma radiation approaching deterministic noise, therefore, commonly used FFT analysis does not produce satisfying results. The autocorrelation function finds repetitive sequences in the signal, which are related to the oscillation of the weld pool. Changes and irregularities in the frequency of these oscillations indicate weld defects and overall stability of the welding process. A series of welding experiments were studied. After the experiments, the autocorrelation functions were evaluated. Changes in frequency during the weld were related to the geometric characteristics of the weld.