The various mesophasic transitions for an asymmetric polystyrene-block-polyisoprene (PS-b-PI) in film geometry, like the order-to-order transitions, were investigated by in situ grazing incidence small-angle X-ray scattering (GISAXS). Compared with the sequential phase transitions in bulk, lamella (LAM) to hexagonally perforated layer (HPL) to gyroid (GYR) to hexagonally packed cylinder (HEX) to disorder (DIS) by thermal energy, the only difference in geometry leads to the enhancement of LAM phase, the parallel shifts of transition temperatures for HPL and GYR phases, and a little increase of order-to-disorder temperature (T-ODT) as a consequence of weak interfacial interactions. However, a short-range of HEX phase in bulk disappears. These results may be correlated to the suppressed compositional fluctuation in film geometry due to the interactions of polyisoprene block with the substrate and air.