using(SPSite site = new SPSite(“http://portal”))

{

foreach(SPJobDefinition job in site.WebApplication.JobDefinitions)
{
if(job.Name == “Workflow Auto Cleanup”)
{
job.Execute(new Guid(“C9CBB265-FB94-497f-8AC0-E99CDFBC4F52″));
}

}

}

来源: http://www.evrenayan.net/post/How-to-Execute-Timer-Jobs-Programmatically.aspx