Cyperaceae (sedges) and Poaceae (grasses) are monocotyledonous plants that exhibit similar morphological characteristics, including reduced flowers, bracts subtending the flowers, and solitary flowers. The objective of this study was to gather data on the diversity of Cyperaceae and Poaceae in Rong Kham Luang forest park, Phayao province. Data collected include common names, flowering periods, and ecological information. Field surveys and specimen collection were conducted between October 2023 and October 2024 in the vicinity of the park office and along nature trails. Four genera and 11 species of Cyperaceae were recorded and the key to species was provided. Cyperus L. was the most species-rich genus, with six species, followed by Carex L. and Scleria P.J. Bergius with two species, and Fimbristylis Vahl with one species. Cyperaceae were most frequently observed in open areas, with peak flowering occurring between May and September. For Poaceae, 13 tribes, 37genera and 46 species were identified including the key to species. Setaria P. Beauv. and Panicum L. were the most diverse genera, each comprising three species. The Poaceae were primarily foundin open areas, dry dipterocarp forest, and mixed deciduous forest, with thepeak flowering between November and December. Additionally, Panicum laxum Sw., a grass species that had not been previously documented in Thailand. This studycontributes the understanding the Cyperaceae and Poaceae in Thailand and establishes a foundational reference for them in Rong Kham Luang forest park.