How to locate your serial number
To ensure that your bike can be tracked back to you in the case that it is stolen, the police need to have distinct identification information such that it can be proved that you are the owner of the bike. The best way to identify a bike and prove ownership is to record the serial number.
Most commonly, the serial number can be found printed on the underside of the bottom bracket where the two pedal cranks meet To find the serial number, turn your bike upside down and look in the area as described in the diagram below.
Image courtesy of Auckland Transport
If you find that there is no serial number near the cranks, there are a number of other common locations you can check including the headset, rear stays, seat downtube and the top of the crank.