In this paper, I point out certain features of participles (often called "verbal nouns" in Mongolian linguistics) in The Secret History of the Mongols (SHM) which was written in Middle Mongolian. Specifically, I show that: I) Most participles except for imperfective and non-past lack the predicative use. II) The predicative use of non-past participles (V-QU) is i. only found within discourse; and ii. mainly found in interrogative, negative, and conditional sentences, i.e., it is only used with modal features that carry the speaker``s modality, such as deontic or epistemic meanings. III) The predicative use of non-past participles in SHM seems to share these features with the predicative use of the rentaikei (adnominal) form in Old Japanese. IV) These aspects correspond with certain typological features found in neighboring Siberian languages, including certain modern Mongolic languages.