The best way to enter Bhutan is by Druk Air, the only national air carrier. Following stations are connected with Druk Air flight:
From India: Delhi & Calcutta
From Nepal : Kathmandu
From Thailand: Bangkok
Druk Air is the only Airline operating in Bhutan.
Entry or exit is also possible through Phuntsoling, the southern Bhutanese border town. From here the nearest airport is Bagdogra in West Bengal (India). For travelers wishing to visit Indian states of Sikkim, West Bengal along with Bhutan, Phuntsholing serves as the convenient point.
Exit from Bhutan can be also made through Samdrup Jongkhar, an important link for visiting further Northeastern states of India.
AS PER GOVERNMENT RULE IT IS MANDATORY TO TRAVEL AT LEAST ONE WAY BY DRUK AIR, IT CAN BE EITHER ENTRY OR EXIT OR BOTH.