KTiOPO-4(KTP) crystal is a nonlinear optical material with superior properties. The KTP structure, point group mm2, is typified by the chemical form {K++}〔Ti 4+〕O(P 5+)O-4, where the curly brackets indicate nine or eightfold coordination, the square brackets sixfold octahedral coordination and the parentheses fourfold tetrahedral coordination. Different isovalent and aliovalent substitutions in a given structure is quite interesting. The KTP-type structure is extremely accommodating with respect to partial or complete isomorphous substitution which may be especially significant in optical devices such as waveguides. A monographic review of the structure-property relationship of KTP isomorphous substitution is briefly and precisely presented. It is shown that the properties of KTP are very sensitive to its structure. Among the composition, structure and properties of KTP-type crystal, the structure is the core because it is a link between the composition and properties.