The phase behavior of polystyrene-block-poly(n-pentyl methacrylate) copolymers [PS-PnPMA] having various block length ratios was investigated by rheology, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). All of the PS-PnPMA exhibited two transitions: the lower disorder-to-order transition (LDOT) and upper order-to-disorder transition (UODT) occurring at a lower and higher temperature, respectively. Thus, PS-PnPMA is characterized by a closed-loop type of phase behavior. As the volume fraction of PS in PS-PnPMA was decreased from 0.5 to 0.23, the microdomains observed between the LDOT and UODT changed from lamellae (LAM) to hexagonally packed cylinders (HEX) and finally to body-centered-cubic spheres (BCC), similar to other block copolymers exhibiting only an order-to-disorder transition. The rheological properties depended strongly on the type of microdomains within the closed-loop. The storage and loss moduli (G' and G") within the closed loop are the highest for a PS-PnPMA having a BCC morphology, then HEX morphology, and the lowest for LAM morphology.