InP quantum dots were grown by using the molecular beam epitaxy technique. The quantum dots were connected and composed a string-like one-dimensional structure in the [1–10] crystal direction due to the strain field along the [110] direction. Two prominent photoluminescence transitions from normal quantum dots and string-like one-dimensional structures were observed and showed strong optical emission anisotropy. Both peaks also showed blue-shifts while rotating the emission polarization from the [1–10] to the [110] direction. Such optical transition behaviors are the consequence of valence band mixing caused by the strain field along the [110] crystal direction.