Abstract. Response of the ion thermal diffusivity ( ) to the radial electric field ( Er) shear has been investigated in JT-60U and the following results were found. (1) In the case of positive magnetic shear (PS) plasma, in the core region shows L mode state, weak internal transport barrier (ITB), and strong ITB depending upon the heating power. In the case of reversed magnetic shear (RS) plasma, however, no power degradation of is observed. (2) In the case of weak ITB, decreases gradually with increasing the Er shear for both PS and RS plasmas. There exists a threshold of an effective Er shear to change its state from weak to strong ITBs. (3) The threshold of the effective Er shear in the case of RS plasma is small compared with that in the case of PS plasma.