Nakuru police commander Hassan Barua said the driver of the bus was overtaking a truck when he collided with an oncoming truck.
Barua said 18 people died on the spot in the accident that occurred at 2.30am.
"One died as he was being rushed to the hospital while the other died while undergoing treatment at the hospital," he said.
He said that several others were injured during the incident and are admitted to St Mary's Hospital in Nakuru.
"We are investigating the matter because the bus is supposed to be plying on Nairobi - Githurai 45 but was carrying passengers to Western hence an offence as per the traffic rules," he said.
This comes barely a month when 25 people died in another road accident at Kambuu, Kibwezi sub-county, Makueni.
Area OCPD Leonard Kimaiyo said the accident involved a bus and a tanker transporting vegetable oil.
The bus, which was on its way to Nairobi from Mombasa, collided with the tanker killing the 25 instantly.
-The Star