Since your parents were at one point paying the mortgage, that is considered to be a form of rent. Therefore they have a "periodic tenancy" (i.e. they are month-to-month tenants) even though there is no formal lease. Accordingly, you will need to serve them a 30 day notice to quit the premises. If they fail to move within that time, you will need to file an "unlawful detainer" (i.e. eviction lawsuit). At that point, once a judge issues an writ of possession (or your state's equivalent), you can request that the sheriff remove them if necessary.In the meantime do not be tempted to use any self-help measures such as changing the locks, turning off the utilities, or removing their belongings. You could be sued for unlawful eviction if you do. Bottom line, you should consult with an attorney in your area who specializes in landlord-tenant cases.