We investigated thin films obtained from the blends of PS-b-PEO diblock copolymer and PS homopolymer (PS homo) at the air/water interface to reduce the laborious work of synthesizing various block copolymers for each specification of thin films. We controlled total PS wt % and α defined as (Mn of PS homo / Mn of PS in block copolymer) by changing amount and molar mass (Mn) of PS homo respectively, then we drew the morphology diagram with these two variables. Generally, expansion of domain size and morphology transition from circular to cylindrical to planar morphology were observed as the amount of PS homo increased, and a similar transition was observed when Mn of PS homo goes up. New morphology that we call melting down morphology in this paper was found at small α, presumably due to the low glass transition temperature of PS homo. At last, we propose a possible blending mechanism of PS-b-PEO and PS homo at the air/water interface based on our results.