Domain structures and their reversal process of (010)-cut Pb(Mg1/3Nb2/3)O-3-38%PbTiO3 single crystal were in situ investigated under varying electric fields by polarized light microscope. High-density needlelike ferroelectric microdomains were observed in macrodomains. When an electric field along [101] was applied on the crystal, macrodomain walls keep immobility, while the vertical microdomains first flip to the transverse direction and then to the vertical direction again. The two-step 90 degrees domain reversal exhibits quite different characteristics. The first step finished quickly at a low electric field and in a narrow electric field range, leading to high polarization current. The second step happened slowly at the high electric field and in a wide electric field range, and no obvious polarization current was observed. Two-step domain reversal mechanism was investigated, and our findings will help in understanding the excellent properties of relaxor Pb(Mg1/3Nb2/3)O-3-PbTiO3 single crystal.