Udom Puasakul has decided to quit as TOT chairman after only nine months in the role, saying he has failed to adequately perform his job.
Mr Udom said he had evaluated his own performance at the state-owned telecom company and could only give himself "four out of 10".
Mr Udom had previously served as director-general of the Department of Public Works and Town and Country Planning. He is seen as having a good relationship with Yaowapa Wongsawat, a sister of ousted prime minister Thaksin Shinawatra.
"I made the decision myself and will inform the information and communications technology minister. The resignation will be effective immediately after the minister receives the letter," he said.
Mr Udom said he had decided to quit three months ago because he was not happy with his own performance in the first six months of his tenure.
"I take my performance seriously. So it's time to leave and give a chance to a new person who will be appointed to the post," he said.
The chairman said he was proud that the phase-one expansion of TOT's 3G network would be completed under his board after several years of delay.
Samart and Loxley, which won the phase-one contract to expand TOT's 3G network, have completed the construction of 4,900 base stations.
The firms are expected to complete all 5,200 base stations and hand them over to TOT next month.
The two contractors had previously complained about work delays, citing TOT's inability to find suitable locations for them to build the base stations.
TOT plans to construct an additional 8,000 base stations for the 3G network under phase two, as well as other base stations for its 4G network.
Mr Udom said TOT must find the right direction to generate sustainable income in its new role as a network provider.
The state-owned firm should aim to operate mainly as a network provider, leaving 3G or 4G services to mobile virtual network operators (MVNOs), he said.
An MVNO is a company that provides mobile phone service but does not have its own licensed frequency allocation or infrastructure.
TOT has five companies operating 3G services as MVNOs as part of its phase-one network.
From Dec 20 this year, however, TOT will no longer be able to book revenue shared under concessions on its financial statements, as per the Frequency Allocation Act of 2010.