Barium titanate (BaTiO3, BT) and potassium niobate (KNbO3, KN) ceramics were prepared by normal sintering process. Phase diagram of BT-KN system was investigated using high-temperature XRD and dielectric measurement, and two phases coexistence between tetragonal and orthorhombic was observed at room temperature. Transmission electron microscope (TEM) observation confirmed that the microstructure of the ceramics with KN contents below 30 molar% was composed of only tetragonal BT-rich grain, while those with KN contents above 40 molar% were composed of two kinds of grains, i.e., tetragonal BT-rich and orthorhombic KN-rich grains. Phase boundary between two phases for BT-KN ceramics was grain boundary. Thus, we named two phases coexistence region for BT-KN ceramics “wide-band morphotropic phase boundary (MPB)”. Finally, dielectric and piezoelectric properties were investigated, and both dielectric and apparent d33 maximum were observed at the 0.5BT-0.5KN ceramics.