Diffun, the gateway to the province of Quirino, is one of the six (6) municipalities of Quirino. Diffun was created as a legal and corporate entity on July 2, 1950 during the incumbency of congressman Leon Cabarroguis and Governor Marcelo Maddela of Nueva Vizcaya.

Diffun is formerly a sitio of Barrio San Isidro, Santiago, Isabela since 1932. it has been the center of hunting ground among the natives and the Ilocanos. It was a primeval forest with many scattered rolling hills up and down.

Diffun was created as a municipality with the  issuance of  Order #466,S-1950. It is situated in the northwestern  portion of the province, bounded  on the north by Cordon Isabela and Santiago City; on the south by Kasibu, Nueva Vizcaya; on the west by Bagabag and Diadi, Nueva Vizcaya; on the east by Cabarroguis and Saguday, Quirino.

The municipal landscape is characterized by relatively flatlands punctuated at certain points by undulating and rolling terrain, with mountain ranges flanking its western and southern sides (i.e. Namamparan Mountains). The low lying hills and surrounding mountain  ranges are slightly dissected by intermittent creeks and water ways (i.e Ganano River) which act as a natural drain of accumulated rein off water coming from the upland.

The municipality of Diffun has a total area of 21,990 hectares of forest land comprising 62.40% of the total land area, although most of the forest is gone, already denuded due to unscrupulous and unlawful logging in the past. The predominance of  forest vegetations in high land areas resulted in manageable  erosion problems for Diffun. Erosion prone areas are only those parts of town along the banks of Ganano River. This situation, however, is not alarming.

The climate is characterized with seasons not very pronounced but it is relatively dry from November to April and wet during the rest of the year. Typhoons usually occur during the third quarter of the year with an average of 4-10 typhoons per year. Floods pose no major effect in the municipality considering its rolling terrain that facilitates drainage of run-off  water.

Diffun has several bedrock formation, the first of which is mainly limestone. However, it must be noted that there is a fault line and zone which is located at the boundary of the municipality of Cabarroguis in the south.