This paper discusses the ocean—continent transition of the magmatic arc and its processes in terms of the igneous petrotectonic assemblage, the geologic characteristics, and the arc maturity. The major igneous petrotectonic assemblages characterized the oceanic subduction may be tonalite—trondhjemite—granodiorite (TTG), high Mg andesitic, Mg andesitic, adakitic and Nb enriched arc basaltic assemblages. Based on the disposition of the petrotectonic assemblages, four possible models of oceanic subducted crust—mantle structure are discussed: (1) the warm young subducted oceanic crust and the above situated cool mantle wedge lithosphere; (2) both the cool older subducted oceanic crust and the mantle wedge lithosphere; (3) the cool older subducted oceanic crust and the above situated warm mantle wedge asthenosphere; (4) both the warm oceanic crust and the mantle wedge asthenosphere. The tectonic indications of the arc magmatic front as the structure markers and the special compositional polarity are also discussed. A model of two layered arc crust is suggested, the lower arc crust may be composed of the mafic granulites and amphibolites, and the upper crust consists of the felsic TTG gneisses. The petrologic structure of the arc crust may have a typical transitional character between the ocean crust and the continental crust or may be regarded as the first stage of the continental crust, the juvenile continental crust. The magmatic arc and the transitional character of the arc crust are regarded as the most important record for the ocean—continent transitional belts (or the accretional orogenic belts) derived by the oceanic subduction.