The Mae Ngat Somboonchol Dam, located in Chiang Mai Province of northern Thailand, was constructed to serve the purposes of irrigation and electricity generation for the region. This hydroelectric dam is located on the Mae Ngat River, which is a tributary of the Ping River. The aim of this experiment was to study the diversity and abundance of the population of caddisflies in this river. Seven sampling sites, including 2 upstream and 5 downstream sites, were monitored monthly for adult male caddisflies using a light pan trap device over 1 year (June 2013 – May 2014). A total of 3,844 male specimens belonging to 16 families and 94 species were caught. However, the total abundance of caddisflies was higher at the upstream sampling sites than at the downstream sampling sites. The top three most abundant species found to be present in this river were Oecetis scutulata MARTYNOV, 1936, Setodes argentiguttatus GORDON & SCHMID, 1987 and Cheumatopsyche globosa ULMER, 1910, respectively. The species compositions of the upstream and downstream sites differed. 44 specimens of the species Trichoptera were represented exclusively at the upstream sites. Examples of this include Macrostemum fenestratum ALBARDA, 1887, Setodes endymion MALICKY & CHANTARAMONGKOL, 2000, Hydropsyche camillus MALICKY & CHANTARAMONGKOL, 2000 and Ecnomus jojachin MALICKY & CHANTARAMONGKOL, 1993. The 23 species exclusively represented in the downstream sites included Aethaloptera sexpunctata KOLENATI, 1859, Cheumatopsyche banksi MOSELY, 1942, Oecetis tripunctata FABRICIUS, 1793 and Ecnomus votticius MALICKY & CHANTARAMONGKOL, 1993. There were 50 newly recorded species while 18 species were absent when compared to a previous study from 2009. The diversity and abundance also differed during each month of collection. The highest numbers of trapped male caddisflies were found from April to May, which were 352 and 292, respectively. The upstream sites contained a higher number of caddisflies and a greater level of diversity than the downstream sites.