They can be synonyms when meaning "cause" as a noun. But, when used as verbs, they are pretty different. Reason can also mean "logic" or "thought." When used as a verb, it basically means "think" A factor is something that makes a whole when combined with other things. It is used in math. For instance in the equation a*b=c, a and b are "factors." When used as a verb, it is often used with "into." For instance: "Location should factor into your decision when buying a house." Even when used as nouns, there are slightly different connotations and uses. For instance, it is common to say something like "Experience is a factor in our hiring process." But you wouldn't say "Experience is a reason in our hiring process." On the other hand, you could say "The only reason I am here is to get a job." But you couldn't say "The only factor I am here is to get a job." Usually there are "reasons for," and "factors in."