A thermocline is a layer of water in which a vertical temperature gradient is observed. On freshwater lakes, it is caused by seasonal warming of the surface layer. In the absence of wind mixing, the upper water layer is heated relatively uniformly, the temperature in the lower layer does not change. The temperature transition between the surface and bottom layers creates a thermocline. Since a change in water temperature leads to a change in its electrical conductivity and dielectric constant, a thermocline can be detected using ground penetrating radar (GPR), a method based on recording short electromagnetic pulses reflected from the boundaries of layers with different electrical properties. That is why GPR can be effectively used in hydrological studies. Studies of thermocline with GPR have been conducted for several years on freshwater lakes in Karelia, Russia. The OKO-2 instrument with a 150 MHz antenna unit was used, which was placed in an inflatable boat. Data on the water temperature were collected by a special thermistor chain at the same time as the GPR data. A layer with a significant temperature gradient was detected, which gives clear reflections on radargrams, its fine structure and changes with time were investigated. The article presents the results of field measurements, as well as the physical and numerical modeling of the propagation of GPR signals in the gradient layer.