Drivers of red public songthaews and tuk-tuks in Chiang Mai submitted a petition to provincial governor Supachai Iamsuwan to complain about the Grab ride-hailing app.
The service is offered on smartphone apps and offered by normal drivers.
The petition said the app had caused their income to drop by more than 50 per cent.
Deputy governor Phutthipong Sirimat said he would pass this case to the Dhamrongtham Center for further investigation.
Phutthipong said the director of the provincial land transport office would be contacted to discuss the issue.
The group's leader, Korn Kankanokporn, said drivers’ incomes had dropped from around Bt2,500 per day to less than Bt1,500 since Grab arrived.
He said Grab used personal cars and motorcycles, which had no public transport licence and were hence illegal.
The group has made similar complaints before without success.
He said his group would employ the Chiang Mai Lawyers Council.