  If neutrinos are Dirac fermions, certain new physics beyond the standard
model should exist to account for the smallness of neutrino mass. With two
additional scalars and a heavy intermediate fermion, in this paper, we
systematically study the general mechanism that can natrally generate the tiny
Dirac neutrino mass at tree and in one-loop level. For tree level models, we
focus on natural ones, in which the additional scalars develop small vacuum
expectation values without fine-tuning. For one-loop level models, we explore
those having dark matter candidates under $Z_2^D$ symmetry. In both cases, we
concentrate on $SU(2)_L$ multiplet scalars no larger than quintuplet, and
derive the complete sets of viable models. Phenomenologies, such as lepton
flavor violation, leptogenesis, and LHC signatures are briefly discussed.
