Serial number?
If so, what kind of frame do you have? Brand? Material?
Most better quality frames were and are serialized. The location varies. Look for the S/N on the bottom bracket shell area and the rear drop outs. Some manufacturers used the head tube or head tube lugs and at least two used the seat stay cluster lug.
With the newer carbon fiber frames the S/N can be located almost anywhere and it may be duplicated on one of the several bar code stickers applied to track the frame thru manufacturing, shipping and sales.
Not all frames were serialized, back in the day. If your frame is an inexpensive steel frame or even an older high end frame it may not have a S/N.