More than thirty species of radiolarians are described from maroon chert and siliceous shales from the Den Chai and Santisuk sections in the east of the Sukhothai fold belt. The fauna obtained from Den Chai comprises Muelleritortis cochleata cochleata, M. cochleata tumidospina, M. expansa, Triassocampe deweveri, T. coronata, T. scalaris, Annulotriassocampe companilis, A. multisegmantatus, A. sulovensis, Pseudostylosphaera coccostyla coccostyla, P. imperspicua, Canoptum inornatus, C. levis, Corum kraineri, Spongoserrula rarauana, Orbiculiforma karnica and others. The assemblage may be correlated with those reported in the Fang-Chiang Dao and Lumphun areas in the north of Thailand and in the ChangningMenglian belt of western Yunnan, South China and suggests a late Ladinian(Middle Triassic) age. The radiolarian fauna from the Santisuk Section comprises Muelleritortis cochleata cochleata, M. sp., Striatotrissocampe nodosoannulata, Triassocampe. deweveri, T. sp., Pseudogodia? sp., Pseudostylosphaera coccostyla coccostyla, Archaeocenosphera sp., Annulotriassocampe multisegmantis, Pseudostylosphaera sp., and others which indicates a late Ladinian age. Middle Triassic radiolarian chert and siliceous shale from Den Chai and Santisuk are interpreted as having been deposited in an extensional continental margin within the Sukhothai fold belt and not in the Devonian–Permian back-arc basin of the Nan suture. These deep marine sequences were deposited in a subsiding continental margin in the Lampang-Phrae Basin. We consider these sequences to be part of the Hong Hoi Formation(Anisian–Ladinian) of the Lampang Group which, in turn, may be compared to the Choushui Formation and other associated Middle Triassic formations in the Simao Basin of South China. More than thirty species of radiolarians are described from maroon chert and siliceous shales from the Den Chai and Santisuk sections in the east of the Sukhothai fold belt. The fauna obtained from Den Chai comprises Muelleritortis cochleata cochleata, M. cochleata tumidospina, M. expansa, Triassocampe deweveri, T. coronata, T. scalaris, Annulotriassocampe companilis, A. multisegmantatus, A. sulovensis, Pseudostylosphaera coccostyla coccostyla, P. imperspicua, Canoptum inornatus, C. levis, Corum kraineri, Spongoserrula rarauana, Orbiculiforma karnica and others. The assemblage The radio-larian fauna from the north of Thailand and in the Changning Menglian belt of western Yunnan, South China and suggests a late Ladinian (Middle Triassic) age. The radiolarian fauna from the Santisuk Section Muelleritortis cochleata cochleata, M. sp., Striatotrissocampe nodosoannulata, Triassocampe. Deweveri, T. sp., Pseudogodia? Sp., Pseudosty losphaera coccostyla coccostyla, Archaeocenosphera sp., Annulotriassocampe multisegmantis, Pseudostylosphaera sp., and others which indicates a Late Ladinian age. Middle Triassic radiolarian chert and siliceous shale from Den Chai and Santisuk are interpreted as having been deposited in an extensional continental margin within the Sukhothai fold belt and not in the Devonian-Permian back-arc basin of the Nan suture. These deep marine sequences were deposited in a subsiding continental margin in the Lampang-Phrae Basin. We consider these sequences to be part of the Hong Hoi Formation (Anisian -Ladinian of the Lampang Group which, in turn, may be compared to the Choushui Formation and other associated Middle Triassic formations in the Simao Basin of South China.