In this paper, we propose a passive microscopic fluidic diodes with no moving parts for simple fluids using asymmetric channel structures. Finite element simulations demonstrate that the fluidic diode conducts water flows preferentially in one forward direction while blocks flows in the reverse direction in a wide pressure range. The exceptional rectification performance is owing to the anisotropic direction-dependent activation pressures. In the forward direction, the activation pressure is small, which is controlled by the infiltration pressure of the small channel. In the backward direction, the activation pressure is large due to the high release pressure at the channel exit. The effective working pressure range for the fluidic diode can be flexibly adjusted by modifying the channel size or the surface property. Furthermore, we create a microfluidic diode fabricated on silicon membranes using laser direct writing. The diode achieves flow rectifications in a certain pressure range, which confirms the underlying rectification mechanisms. This work provides a novel strategy for flow control or logic computations in integrated micro- and nanofluidic systems.