The nearest airports are Guwahati and Dibrugarh, if you are traveling to the western part of Arunachal.
Arunachal State Transport runs buses daily from Guwahati to Bomdila and Itananagar. Buses depart from the Assam Interstate Bus Terminal at 6-7 am.
You can also access Bomdila and Itanagar by shared vehicles from Tezpur Assam. One can go to Tezpur by state transport buses. From Tezpur, shared vehicles are available every morning from 5:30 am. Make sure you book a nice seat with a reputed travel agency. Dont forget to request your driver to halt at Sela pass. Its covered with snow during winter seasons with a nice picturesque view. The roads are maintained by Border Road Organization BRO; there is the possibility of land slides in the rainy season.
If you are visiting Namdapha, then the nearest airport will be Dibrugarh Assam. There are no direct flights to Itanagar and the nearest airport is Tezpur. There is a helicopter service available from Gauhati to various destinations like Pasighat and Tawang in Arunachal Pradesh. The service is operated weather permitting. The fares are reasonably priced. There are no public taxis operating at the Helipad in Tawang. If you want to trek from the Helipad to Tawang town make prior arrangements for road transport before boarding the flight.