A statement posted on the board's website on Thursday showed the board decided to launch the probe of Turkcell on Oct. 9.
Government sources told Reuters that Turkcell's campaigns which gave bonus time to subscribers had been the reason for the probe.
"Such statements on the Competition Board's probe decisions are aimed to inform the public and do not mean that the enterprises in question have violated competition rules and will be subject to penalties," the board's statement said.
Turkcell declined to comment when contacted about the probe.
Turkcell's major shareholders are Nordic telecommunications company TeliaSonera, Russian private equity firm Altimo and unlisted Turkish conglomerate Cukurova.
Turkcell's competitors are Avea, owned by Turk Telekom and Vodafone.