The majority of visitors arrive to Mongolia by air through Chinggis Khaan International Airport located 18 km to the southwest of Ulaanbaatar. The airport reconstructed in 1990 currently house the immigration and customs offices. Since the refurbishment, the waiting area has become more comfortable and the luggage delivery service has improved greatly. The international airport is operated by MIAT Mongolian airlines, Korean Air, Air China and Aeroflot while the domestic air transport is operated by MIAT, Aero Mongolian and Hunnu Air companies.
International flights to Mongolia from Moscow, Beijing, Hong-Kong, Berlin, Tokyo, and Seoul. You won't need to worry about travel visas for connecting flights through these countries as long as you are just there for a layover and are not planning to leave the airport.
Air China is the cheapest, but it is prone to delays and cancellations. MIAT Mongolian Airlines, the national carrier, flies to a handful of international destinations. Korean Air is posh and comfortable with the best service. If you have the choice, pay the extra $100 or so and fly Korean Air
There are very few visa requirements for visitors entering Mongolia up to 30 days, but be aware that if you want to stay longer, you must register that plan within the first 7 days or risk exit delays and penalties when you try to leave. You can register only in Ulaanbaatar. Please, see more about visa requirements by clicking here.
Trains come into Mongolia via Russia in the north and China in the southeast. While there are various stops along the Trans-Mongolian line of the Trans-Siberian railway within Mongolia, there is only one point of entry at either border.
Trains coming from Russia enter through Naushki, Russia and trains from China enter though Erlian, China. Unless you have another destination along the line in mind, it would be best to purchase tickets from wherever you are in Russia or China directly to Ulaanbaatar. Obtaining train tickets to either Russia or China from Mongolia is a different process than in-country travel. These tickets are purchased at the Mongolian Rail (MTZ) building across the street from the train station and to your left if you are facing away from the train station.