The Sinhalese language is known to have a SOV word order, but some studies (e.g., Herath, et al., 1994) suggest that word order in the Sinhalese language is more flexible than the Japanese language. The present study investigated priority information among case particles, thematic roles and grammatical functions, for determining canonical order of colloquial Sinhala sentences. Processing grammatically/semantically acceptable colloquial Sinhala sentences for the sentence-correctness task by 38 native Sinhalese speakers showed consistently significant scrambling effects throughout the four experiments. The active sentences of Experiments #1 and #2 supported scrambling effects existing in the colloquial Sinhalese language. In the passive sentences in Experiment #3, thematic roles and case markers offer different information regarding canonical order. Experiment #3 proved scrambling effects in the direction indicated by case particles. Thus, thematic roles were excluded from the priority of information. Since the dative case is assigned to syntactic properties of the subject in potential sentences (e.g., Fukui, 1995; Shibatani, 1985), case particles and grammatical functions provide different information concerning canonical order. Experiment #4 revealed the scrambling effects on potential sentences which were ordered on the basis of grammatical functions. Thus, as Tamaoka et al., (2005) indicated, the scrambling effects in the present study also showed that neither thematic roles nor case particles can provide fully satisfactory information for canonical order, and that only grammatical functions offer plausible information in all active, passive and potential Sinhalese sentences.