Place popcorn in a large bowl. In a microwave-safe bowl, combine the butter, sugar and cinnamon. Microwave on high for 45 seconds; stir. Microwave 45 seconds longer or until the butter is melted. Pour over popcorn and toss to coat.
Transfer to two greased 15x10x1-in. baking pans. Bake, uncovered, at 300° for 10 minutes. Cool. Store in an airtight container.