Tonalite and trondhjemite (plagiogranite) are typical granitoid rocks that contains abundant plagioclase. IUGS defines tonalite as the plutonic rocks that contains more than 20% quartz and plagioclase portion higher than 90% of the feldspar minerals. Meanwhile, the trondhjemite is defined as the light-coloured (mafic minerals, M<10%) tonalite. Tonalite and trondhjemite are the important constitute of Precambrian continental crust, and they also occurred in the plutonic assemblage of arc systems and in the ophiolite suites. The trondhjemites exposed within TTG suites exhibit high aluminous character, but many trondhjemites occurred in ophiolite suites have low aluminous content. Using the QAP classification results discriminated by the actual mineral modes as the benchmark, two chemical diagrams, i.e., SiO₂'-CaO/(CaO+K₂O) plot and 100×(Mg+Fe)/Si-SiO₂ plot are proposed as the tools to discriminate tonalite and trondhjemite from the other granitic rocks. Furthermore, the ACNK-Al/Si plot and ACNK-FeO^* plot are suggested to separate the low-aluminous trondhjemite (LAT) from the high-aluminous trondhjemite (HAT). The case studies indicate that the new chemical procedure performs well for discrimination of tonalite and trondhjemite occurred within the TTG suites as well as those occurred in the ophiolite suites. Avoiding the calculation of normative minerals, the new chemical procedure is accurate, fast, and user-friendly than the scheme base on normative An-Ab-Or diagram, which is commonly used as the key plot to identify tonalite and trondhjemite for many years.