Costolo told the Code Conference that the messaging service is in the "very beginning stages of conversations about what it would look like or feel like," according to the online news site Re/code, which sponsored the event.
He did not elaborate on any plans, the report said.
Costolo told the conference that his recent visit to China allowed him to talk with Chinese companies that advertise on Twitter about ways they can work together.
Twitter is banned in China along with Facebook, but some private researchers say tens of millions of Chinese circumvent firewalls to access the social networks.
(Also see: How Chinese Authorities Use Long-Banned Facebook)
The Chinese social network Weibo, which is often compared to Twitter, launched a public offering in New York last month, raising some $287 million.
When Costolo visited China in March, the US firm said he wanted "to learn more about the Chinese culture and the country's thriving technology sector" but added that "we have no plans to change anything about our service in order to enter the market."
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