Members of the Unite union are staging two six-hour strikes.
The first started today (Friday) at 3pm and will end at 6pm.
The second will take place on Monday during the same hours.
Unite, which represents almost 400 workers, said Yorkshire Ambulance Service had “snubbed” the union’s attempts during the summer to hold constructive talks.
Terry Cunliffe, Unite regional officer, said: “Our members firmly believe that the public deserve a first-class, fully resourced ambulance service - and, as they have shown on a number of occasions since April 2013, they have been prepared to strike to strongly make that case.
A number of strikes have been held during the past 20 months after the two sides could not reach an agreement.
Unite claims its members will go ten hours without a meal break and be forced to work 12-hour shifts under changes to working patterns.
Ian Brandwood, executive director of HR at the Yorkshire Ambulance Service NHS Trust, said: “We would like to reassure members of the public that we have robust contingency plans in place.”