The opening of the 2,298 kilometre (1,428 mile)-line was commemorated by the 9 a.m. departure of a train from Beijing for Guangzhou. Another train left Guangzhou for Beijing an hour later.
China has massive resources and considerable prestige invested in its showcase high-speed railways program.
But it has in recent months faced high-profile problems: part of a line collapsed in central China after heavy rains in March, while a bullet train crash in the summer of 2011 killed 40 people. The former railway minister, who spearheaded the bullet train's construction, and the ministry's chief engineer, were detained in an unrelated corruption investigation months before the crash.
Trains on the latest high-speed line will initially run at 300 kph (186 mph) with a total travel time of about eight hours. Before, the fastest time between the two cities by train was more than 20 hours.
The line also makes stops in major cities along the way, including provincial capitals Shijiazhuang, Wuhan and Changsha.
More than 150 pairs of high-speed trains will run on the new line every day, the official Xinhua News Agency said, citing the Ministry of Railways.
Railway is an essential part in China's transportation system, and the government plans to build a grid of high-speed railways with four east-west lines and four north-south lines by 2020.
The opening of the new line brings the total distance covered by China's high-speed railway system to more than 9,300 km (5,800 miles) — about half its 2015 target of 18,000 km.