We use scanning tunneling microscopy to study the temperature evolution of electronic structure in Ca3Cu2O4Cl2 parent Mott insulator of cuprates. We find that the upper Hubbard band moves towards the Fermi energy with increasing temperature, while the charge transfer band remains basically unchanged. This leads to a reduction of the charge transfer gap size at high temperatures, and the rate of reduction is much faster than that of conventional semiconductors. Across the Neel temperature for antiferromagnetic order, there is no sudden change in the electronic structure. These results shed new light on the theoretical models about the parent Mott insulator of cuprates.
