The electrical properties and thermal stability of In/W Ohmic contacts inn‐type GaAs were studied by analyzing interfacial microstructure using cross‐sectional transmission electron microscopy and measuring the contact resistances by transmission line method. Indium layers with various thicknesses were deposited directly on GaAs substrates, which were kept at room or liquid‐nitrogen temperature. The lower contact resistances (Rc) were obtained when the contacts were prepared at liquid‐nitrogen temperature. These lowRcvalues were due to formation of large‐areal InxGa1−xAs phases on the GaAs substrate after annealing at temperatures higher than 600 °C. The In layer thicknesses of the In/W contacts prepared at liquid‐nitrogen temperature strongly affected the contact resistances as well as the thermal stability after contact formation. The optimum In layer thickness which provided the best electrical properties and thermal stability was determined to be 3 nm. The In(3 nm)/W contacts yieldedRcvalues less than 0.2 &OHgr; mm and theRcvalues did not deteriorate after annealing at 400 °C for more than 20 h. The contacts with In layer thicknesses thinner than 3 nm resulted in higherRcvalues due to insufficient InxGa1−xAs phases at the metal/GaAs interfaces. The contacts with In layer thicknesses thicker than 3 nm resulted in poor thermal stability due to formation of large amounts of In‐rich In(Ga,As) phases with low melting points. The present In(3 nm)/W Ohmic contacts are believed to be the simplest metallurgy with excellent electrical properties and thermal stability among In‐based Ohmic contacts.