The announcement comes with tensions mounting overterritorial disputes between Beijing and its neighbours in the East and SouthChina seas.
The Y-20 is the country's largest homegrown transportaircraft, a statement on the defence ministry's website said.
The plane is intended for moving troops and cargo over"long distances in diverse weather", it added.
Reports from its 2013 debut said the Y-20 has a maximumpayload of 66 tonnes, which it can carry as far as 4,400 kilometres (2,700miles).
With 55 tonnes on board it could fly from western Chinato Cairo.
At the time, experts told AFP that the true figures forthe Y-20's maximum load and flying range were likely to be lower than thosecited in state media due to the plane's reliance on a "very old"Russian-designed engine.
The plane's entry into service "marks a crucial stepfor the air force improving its strategic power projection capability,"the official Xinhua news service quoted PLA Air Force spokesman Shen Jinke assaying.
For decades, China stuck to a strategy articulated byformer paramount leader Deng Xiaoping, of "hiding its strength and bidingits time".
But in recent years, it has conspicuously expanded itsmilitary's reach and taken a more aggressive stance on disputes on itsperiphery.
The construction of artificial islands capable of hostingmilitary facilities in the South China Sea has raised tensions with rivalclaimants to regional waters, most notably Vietnam and the Philippines.
It has also earned the ire of the United States, whichsays a military buildup in the area will threaten the free transit ofstrategically important international waterways.
At the same time, China has also played a significantrole in international policing efforts.
China is a major contributor to UN peacekeeping and hasalso assisted combatting piracy in the Gulf of Aden.
In November last year, it announced it would establish anaval facility in Djibouti, the strategically vital entrepot on the Horn ofAfrica.
The new aircraft will be used for"safeguarding national security as well as domestic and internationalrescue and relief work", Xinhua quoted Shen as saying.
Jordan3 hours ago
The tail looksexactly like a C-17. Chinese cannot design anything on their own. Lockheed and Airbushave a leak somewhere. Cross between a C-17 and a AM-400 by the looks of it.The range is long enough to get to the Horn of Africa where they have acquireda new overseas base.
fuka3 hours ago
What a beautifulbird!
Matt3 hours ago
With refueling inthe air, they can reach anywhere on earth without landing and fly back toChina.!
fuka4 hours ago
Well done. Chinais amazing! Those stupid Indian can only buy from Uncle Sam.
o4 hours ago
Great Chinaraising! Nice aircraft to punish Americans' greedy @sses!