A fabrication method of a wafer level semiconductor package includes: forming on a carrier a first dielectric layer having first openings exposing portions of the carrier; forming a circuit layer on the first dielectric layer, a portion of the circuit layer being formed in the first openings; forming on the first dielectric layer and the circuit layer a second dielectric layer having second openings exposing portions of the circuit layer; forming conductive bumps in the second openings; mounting a semiconductor component on the conductive bumps; forming an encapsulant for encapsulating the semiconductor component; and removing the carrier to expose the circuit layer. By detecting the yield rate of the circuit layer before mounting the semiconductor component, the invention avoids discarding good semiconductor components together with packages as occurs in the prior art, thereby saving the fabrication cost and improving the product yield.