A chemical etching process to obtain a double heterostructure of InP‐GaInAsP‐InP with a very narrow active layer width is proposed in which three layers are etched preferentially. The quaternary layer works as the inner etching mask; the cladding layers above and below the active layer show reversed‐mesa and normal‐mesa shapes, respectively. The method is used to fabricate a buried heterostructure laser which lases at 1.3 &mgr;m on thep‐type InP substrate. The fundamental lateral mode operation with threshold current as low as 15 mA is achieved with 0.8‐&mgr;m active layer width.