Dog fleas are a punishment on dogs as they slowly make the pet very ill, so ill that the pet cannot do anything.
I don't think you can use it to kill fleas but you can certainly use it to deter them. Fleas don't like the smell of it at all and will avoid fur that has been brushed with peppermint oil. Also the cooling effect of things like menthol, tea tree oil, and peppermint oil can be really good at relieving the burning or itching that comes from previous bites.
Be VERY careful when using essential oils on pets. Remember to dilute them first. You can mix it with other oils like jojoba or witch hazel with regular dog shampoo during bathing. Do this once or twice a week. Another alternative is to use peppermint soap which works just as well. I personally add a half cup of lemon juice to the petfancy.com dog shampoo i use on my pet. Works good and makes him smell really fresh.