This paper studies the syntactic properties of two Chinese function words ba and bei from a Lexical-Functional Grammar (LFG) perspective. It is proposed that they be identified as multi-valence prepositions or co-verbs that share the argument-taking characteristic of verbs and play the role of syntactic head (vs. semantic head) in organizing a sentence and determining the unexpressed grammatical functions (GFs) in a post-ba/bei XCOMP. The grammatical mechanism conditional functional control (CFC), extended from functional control (FC) in classic LFG, is developed for ba/bei constructions and the CFC rules are accordingly formulated. It is shown that FC not only lies in a control word’s lexical properties but also observes grammatical context . Causativization and passivization by ba and bei , respectively, are unified into FC as special cases of their lexical properties. Previous treatments of bei with no OBJ as verbal prefix is also criticized in the discussion of anaphoric control in bei constructions. The LFG analysis of ba and bei is highly extendable to many other Chinese function words and can significantly facilitate Chinese sentence parsing.