In a land area of 52,283 square kilometers, the diocesan territory covers 10 civil districts in western Arunachal Pradesh.
Itanagar is the biggest town in the diocese. The state borders China, Tibet, Bhutan and Myanmar and comprises snow-capped mountains of the Himalayas and river valleys.
In Itanagar diocese, the population is 898,000 at end of 2016. Adi, Aka, Apatani, Hill Miri, Monpa and Nyishi are the major ethnic groups.
Hindi is widely spoken. Adi, Aka, Apatani, Hill Miri, Monpa and Nyishi are the languages used in the diocesan territory.
The Catholic Church in Itanagar has a unique history. The Indian government, unhappy with the spread of Christianity among the hill tribes of the north-east, decided to keep the Church out of Arunachal Pradesh.
However, young people from Arunachal who went out of the state seeking good education came into contact with Catholic schools in Assam and Meghalaya.
Father L. Cerato SDB, the parish priest of N. Lakhimpur, accepted several Apatani boys in his school in the early 1960s. He sent some students also to Dibrugarh as N. Lakhimpur was at that time under Dibrugarh diocese. Bishop Orestes Marengo, the ordinary of Dibrugarh, encouraged this apostolate. The first baptisms of Apatanis took place in Lakhimpur in 1963. In 1969 Father Joseph Mittathany became the bishop of Tezpur. He began to encourage admission of Apatani students into various hostels of the diocese. Several students found their way also to other Catholic Institutions in the region especially Shillong and Dibrugarh. Most of these students were baptized and sent back with the mission to evangelize Arunachal Pradesh.
A more significant breakthrough in Arunachal Apostolate came with the establishment of the parish of Harmuty by Bishop Joseph Mittathany in 1977. The parish is located close to Banderdewa, the entry point to Itanagar, the capital of Arunachal Pradesh. Father Kulandaisamy, a charismatic missionary, was appointed the parish priest of Harmuty. His friendliness and welcome attitude brought him many guests from Arunachal. He baptized some of them. Those baptized were given the mission of bringing others to Harmuty.
The installation of the first bishop of Itanagar on March 12, 2006 was a joyful event for west Arunachal. The Catholics celebrated the occasion with a sense of triumph; especially those who had gone through hard times because of the earlier antagonism of the government to the Church.
The big towns are managed by corporations. The villages and small towns are administered by elected local bodies called panchayats and municipalities, respectively.
The nearest airport is in Itanagar city.
Per capita income in the diocesan territory is Rs 25, 836 ($563) as of January 2010. Farming especially agriculture is done by 90 percent of the population. Petroleum and coal mining are also situated in the diocese. The state of Arunachal Pradesh has a big potential for hydro electricity power generation.
Government and private operators provide extensive telecommunication facilities in the diocesan area. The diocese is well connected by local cable TV networks.
Literacy rate in the diocese is 55 percent.