Low temperature transport in multiwalled carbon nanotubes (MWNTs) has been studied at different diameters and lengths, within 2–10 nm, and 0.3–3.5μm, respectively. In a majority of the samples, semiconductivity showed up as a transport gap in the gate voltage controlled conduction, but metallic MWNTs are found in all diameters. The transport gap is seen to be quantitatively determined by a diameter dependent band gap, and length dependent localization of charge carriers. The band gap of semiconducting MWNTs is estimated to be smaller than that extrapolated from the conventional expression applicable to semiconducting single wall carbon nanotubes. The results constitute a systematical study on size dependent transport and especially of semiconductivity in MWNTs.