Place marshmallows and butter in an extra large, microwave safe bowl and heat in microwave on high for 1 1/2 to 2 minutes,or until marshmallows begin to expand.
Mix marshmallows and butter together and then add in Cheerios. Coat Cheerios with marshmallow mixture completely.
Line a 9 x 13 inch baking dish with wax paper and spread Cheerio Treats out evenly in the pan. Sprinkle with rainbow sprinkles.
Refrigerate for at least 30 minutes before serving. Enjoy!