We have fabricated graphene nanoribbons (GNRs) of length up to tens of microns through mechanical cleavage, and performed electron transport measurements on devices made of the GNRs down to a low temperature of 0.3 K. Fabry-Perot interference was observed in the conductance of the devices as functions of both bias voltage and gate voltage. The result indicates that our mechanically cleaved GNRs are of high quality, in which the electrons form an ideal one-dimensional system at low temperatures.