A p-type tunneling field-effect transistor is demonstrated based on a van der Waals vertical heterostructure of WSe2 and MoS2, utilizing the ion gel dielectric as top gate. Band-to-band tunneling is achieved by modulating the band alignment of the heterojunction of WSe2 and MoS2 with gating the WSe2 channel through ion gel top gate. A fabricated tunneling field-effect transistor shows a minimum subthreshold swing of 36 mV dec(-1) and ON/OFF current ratio of 10(6) at room temperature. Furthermore, evidence of band-to-band tunneling is clearly confirmed through temperature dependent I-V characteristics. This work holds considerable promise for the low-power computational devices based on integrated circuits.