With the development of mobile devices

the pen-based input mode has provided the possibility to use and share chemical knowledge easily. As an application of ubiquitous computing

research on online handwritten chemical formulas becomes a hot area. In this paper

we conclude common relations between chemical symbols and segment a chemical formula after linking the broken strokes. Then a two-level classifier is used to recognize the isolated symbols and the formula is understood with some chemical rules. The experimental results show that our method is robust and feasible when used on Tablet PCs.