P-n junction is the main component of many semiconductor devices. Thermodiffusion, ion implantation and molecular beam epitaxy are only a few methods to form a p-n junction. The main drawback for these methods is high cost per p-n junction since the equipment for these methods is expensive. A possibility of p-n junction formation by laser radiation was shown in several p- and n-type semiconductors: p-Si[1,2], p-CdTe[3], p-InSb[4,5], p-InAs[6], p-PbSe[7] and p-Ge[8] due to inversion of conductivity type. Different mechanisms have been proposed to explain the nature of inversion of conductivity type, for example, impurities' segregation, defects' generation, amorphization and oxygen related donor generation. However, the proposed mechanisms have many lacks and even contradictions, therefore the mechanism of p-n junction formation by laser radiation is not clear until now. For this purpose i-Ge crystal was irradiated by Nd:YAG laser with different energy of quantum. I-type Ge crystal was used in the experiments as a model material because the concentration of impurities in this material is lower than the concentration of intrinsic point defects at room temperature. Rectification effect of current-voltage characteristic in pure intrinsic Ge crystal after irradiation by Nd:YAG laser was observed. The effect is characterised by threshold intensity of the laser radiation. Increase of rectification ratio of current-voltage characteristics and barrier height with intensity of the laser radiation, energy of laser radiation quanta and number of pulses was observed in this experiment. The mechanism of this phenomenon is explained by generation and redistribution of intrinsic point defects in temperature gradient field, which causes strongly absorbed laser radiation. The redistribution of defects takes place because interstitial atoms drift towards the irradiated surface, but vacancies drift in the opposite direction * in the bulk of semiconductor according to Thermogradient effect. Since interstitials in Ge crystal are of n-type and vacancies are known to be of p-type, a p-n junction is formed.