This article summarizes our recent studies on rheological properties and structures of polymer blends in miscible and immiscible regions. In the miscible region, viscoelastic properties are quite similar to those of polymer solutions when one of the two components is not entangled and volume fraction of the other component is not so high. That is, viscosity behavior in semidilute region can be explained by the scaling theory and concentration dependencies of elastic parameters are also almost the same as those in solutions. In the immiscible region, shear stress σ and first normal stress difference N₁ of equiviscous polymer blends are proportional to shear rate. Plots of rescaled transient stress curves after step change of shear rate vs. strain compose a single curve when shear rate ratio is constant. These results are consistent with the theory of Doi-Ohta. However, an undershoot of σ and an overshoot of N₁ which cannot be expressed by the theory are observed after a step increase of shear rate. From direct observation of structural change, it was reported that initial ellipsoidal domains were extremely elongated, then ruptured to shorter ones and gradually changed to the final steady-state structure. The behaviors of transient stresses can be qualitatively explained by the change in the distribution of unit normal vectors of the interface. In the immiscible region close to the phase separation point, a rather sharp change of shear rate dependence of N₁ from that in immiscible region to that in miscible region due to shear-induced homogenization was observed.