L1 plays a role in neural development. However, it remains unclear how L1 plays this role. In the present study, we have shown extensive outgrowth of long neurites in cerebellar neurons after treatment with either L1 or L1 antibody. Notably, the mRNA level of FGF21 was significantly increased in both L1 and L1 antibody treated neurons compared to control group. Consistently, the neurite outgrowth promoted by L1 was strongly inhibited by siRNA against FGF21 gene or a treatment of cells with FGFR inhibitor. These results demonstrate that FGF21/FGFR signaling promotes the neurite outgrowth in a L1-dependent manner.