In a statement, Dtac attributed the sharp decline to the expansion of network, prepaid handset subsidisation, higher sales and administrative expenses, as well as unrealised foreign exchange loss.
Like the other two mobile operators, Dtac also offers its own mobile phone brands as well as special data packages.
In the quarter, its revenue rose only 0.2 per cent from the same period last year, to Bt21.9 billion.
At the end of June, it had 26.9 million subscribers, of which 87 per cent subcribing to its Dtac TriNet network. Fifty seven per cent of the subscribers used smartphones.
To serve the increasing demand for data services, Dtac plans the investment of Bt18-Bt20 billion this year to improve and expand its netwroks. It targets to achieve 95 per cent population coverage and largest bandwidth on 4G service by the end of 2015.
"The economic recovery pace in the first half was slow while competition among operators continued to suppress the full growth potential, particularly on handset subsidies and data monetisation" the company said. As such, the service revenue in 2014 is expected to fall below last year's level. Meanwhile, the EBITDA margin is set in the range of 31 - 33 per cent.