We have investigated the tunneling I-V characteristics of Al 0 . 3 Ga 0 . 7 As/GaAs MQW diodes with different electron densities, N s . Clear periodic negative differential resistances (NDRs) are observed for the samples with lowN s , while NDRs are completely absent in the first plateau region of the I-V characteristics of the samples with highN s . It is found that such appearance/disappearance of periodic NDRs in the first plateau region of the I-V characteristics is controlled by the interplay of the transmission probability and the electron supply-function.