It is shown that using an ESR spectrometer with magnetic field modulation and sweeping the temperature across Tc (at a constant and a very low magnetic field), is equivalent to temperature modulation. The signal intensity obtained when crossing Tc is proportional to 1/( delta Hc2/ delta T) at T=Tc. Using the WHH relation Hc2(T=0)=0.7 Tc( delta Hc2/ delta T)T=Tc enabled the measurement of the relative angular variation of Hc2 in single crystals of YBaCuO with Tc approximately 85 K. The data fit the Ginzberg-Landau theory. This very sensitive technique can be used to characterize properties of high Tc superconducting materials. Results on thin films is also be presented.