Oviraptorosauria and Scansoriopterygidae are theropod clades that include members suggested to have partially or fully herbivorous diets. Obligate herbivory and carnivory are two ends of the spectrum of dietary habits along which it is unclear how diet within these two clades might have varied. Clarifying their diet is important as it helps understanding of dietary evolution close to the dinosaur-bird transition. Here, diets are investigated by conventional comparative anatomy, as well as measuring mandibular characteristics that are plausibly indicative of the animal’s feeding habit, with reference to modern herbivores that may also have nonherbivorous ancestry. In general, the skulls of scansoriopterygids appear less adapted to herbivory compared with those of oviraptorids because they have a lower dorsoventral height, a smaller lateral temporal fenestra, and a smaller jaw-closing mechanical advantage and they lack a tall coronoid process prominence. The results show that oviraptorid mandibles are more adapted to herbivory than those of caenagnathids, earlydiverging oviraptorosaurians and scansoriopterygids. It is notable that some caenagnathids possess features like an extremely small articular offset, and low average mandibular height may imply a more carnivorous diet than the higher ones of other oviraptorosaurians. Our study provides a new perspective to evaluate different hypotheses on the diets of scansoriopterygids and oviraptorosaurians, and demonstrates the high dietary complexity among early-diverging pennaraptorans.