The La5/8Ca3/8Mn1-xCoxO3 (0≤x≤0.3) has been systematically studied by means of positron annihilation technique and X-ray diffraction method together with the transport properties measurement. The results show that all the samples have orthorhombic pervoskite structure

and with the increase of Co doping concentration x

the peak value of resistivity ρp increases quickly

and the corresponding metal-insulator transition temperature Tp decreases. The positron annihilation parameters τ1

τ2 and I2 show strong x dependence. The shorter lifetime τ1 and longer lifetime τ2 show a decreasing trend with the increase of x in totally

while the longer lifetime intensity I2 shows an increasing trend. The local electron density ne calculated from positron lifetime parameters has an increasing trend with the increase of x. Together with the transport properties

it could be confirmed that with the increase of Co doping concentration the electron localization effect is strengthened gradually. The Co doping at Mn site causes the conduction network of Mn3+ – O – Mn4+ partly destroyed. It also enhances the electron – phonon coupling effect

then causes the number of itinerant electrons to decrease and the number of electrons in localized state to increase.