We have grown self-organized InGaAs/GaAs quantum wire (QWR) structures on several kinds of (nnl)A and (nnl)B GaAs substrates by molecular beam epitaxy to optimize the substrate orientation. We observed the most uniform and highest-density corrugation with straight step edges running in the [1 (1) over bar0] direction on the surface of a 3.0-nm-thick InGaAs layer on the (221)A GaAs substrate among the (nnl)A GaAs substrate. The lateral period of the corrugation was 36 nn and the height was 1.8 nm. On the other hand, GaAs surfaces were almost flat. Hence, a nominally 3.0-nm-thick (221)A InGaAs quantum well sandwiched by GaAs layers is a high density and uniform QWR structure due to the lateral thickness modulation. Photoluminescence (PL) from the (221)A InGaAs/GaAs QWRs structure as 12 K was strongly polarized along the wire direction and its polarization degree P[=(I-parallel to-I-perpendicular to)/(I-parallel to+I-perpendicular to)] was 0.20. The PL linewidth was as small as 5.8 meV