Japanese Chief Cabinet Secretary, Yoshihide Suga said it was considering protest measures, including suspending its funding contribution to the U.N. body.
Suga also complained that the UNESCO selection process was not transparent.
"The decision-making process for the Memory of the World is extremely obscure. We were not even allowed access to the contents of the document," Suga said.
The documents include 11 sets of files on the Nanjing Massacre, also known as the Rape of Nanking, ranging from film, photographs and text recorded between 1937 and 1948, Chinese state media reports
Japan invaded China in the 1930s before World War II. The Chinese government says about 300,000 people died during a weeks-long spree of mass killings, rape and looting after the Japanese military occupied Nanjing in late 1937 and 1938.
Japan disputes the scale of the incident.
"China will ensure these valuable documents are protected and circulated, and make them play a positive role in remembering history, cherishing peace, looking into the future and safeguarding human dignity," Chinese Foreign Ministry spokesperson Hua Chunying told Chinese state media outlet Xinhua.
China filed the application
with UNESCO more than a year ago.
The UNESCO Memory of the World Register
was established in 1992 with the aim of preserving documentary heritage. It includes everything from the archives of the Dutch East India company and the diary of Anne Frank to the music of Romantic composer Johannes Brahms.
Objections from Japan
An earlier statement
from Japanese foreign press secretary Yasuhisa Kawamura said, "the government of Japan believes that there are obvious problems with the documents' integrity and authenticity."
It called the inclusion "extremely regrettable" given continuous protests from the Japanese government.
The incident is a persistent flashpoint in Sino-Japan relations. Beijing charges that Tokyo has failed to properly apologize for the wartime atrocity.