The A-site disorder effects on the stability of charge-ordered state in electron-doped manganite R0.4A0.6- MnO3 (where R = La, Pr, Nd, Sm, and A= Ca, Sr, Ba) are investigated by detailed measurements of transport, mag- netism, and specific heat. The robust charge-ordered phase within the prototype sample can be successively suppressed by enhancing the A-site disorder, and finally an atomic-scale spin-glass state is evidenced in the highest disordered sam- ple. However, no ferromagnetic phase can be observed; even the long-range charge-ordered antiferromagnetic phase is fully suppressed, which can be associated with the insuffi- cient eg electron density in such an electron-doped mangan- ite. This revealing is helpful for understanding the origin of ferromagnetic phase in nano-sized or B-site doped charge- ordered manganites with small eg electron density.