A number of services on Saturday and Sunday will be cancelled or reduced, although ScotRail has claimed that 76% of routes will operate as normal.
The industrial action is over the union's concerns about trains operating without guards.
On driver-only-operation trains, the driver is asked to close the doors at stations, instead of a conductor.
ScotRail has said it would ensure a second member of staff was scheduled to be on board every train to assist the driver in emergencies.
However, the company has accepted there are occasions when a ticket examiner does not turn up for work that a train could run with only a driver and Scotrail has said it has union agreement for this.
The union believes the absence of a dedicated guard would make trains less safe.
ScotRail has predicted that this weekend's strike will not affect services to Troon, where the Open golf championship is being held.
RMT union members working for ScotRail have started another two days of strike action.