The grain connectivity calculated using resistivity measurements has been investigated using in-situ PIT-processed tapes. The effective current area (1/F) of the tapes, in cases where commercial Mg powder was used, was only 4.3%. On the other hand, 1/F values of tapes using MgH2 and nano-Mg powders were larger than those using Mg powder. 1/F values of C-doped tapes decreased with increasing amounts of C in cases where MgH2 powder was used. Simultaneous addition of 3-ethyltoluene and SiC were effective in preventing the suppression of connectivity, because the packing density was increased using 3-ethyltoluene.