Middle Triassic radiolarians were obtained from a bedded chert sequence in Chiang Dao, northern Thailand. Sixty-four species were identified into two assemblages, which indicate two stratigraphic levels including lower Illyrian (lowermost upper Anisian) and upper Illyrian (uppermost Anisian). The occurrence of upper Anisian radiolarians in our study indicates the former existence of a relatively deep marine environment in northern Thailand with a seaway between eastern and western Palaeotethys. The stratigraphic units containing bedded chert in this area were formerly dated on the basis of fossil evidence and petrological similarities. However, the strata in the study area are possibly not part of a continuous stratigraphic sequence, but consist of tectonic slices. The fossil age from any slice indicates only the geological age of the given slice, and adjacent slices are often of different ages. Parts of the sequence formerly mapped as Carboniferous and Permian are Middle Triassic in the study area. The sequence covers the Carboniferous to Middle Triassic and can be correlated with the eastern zone of the Changning-Menglian Belt in southwestern Yunnan, China.