The Majorana zero mode (MZM), which manifests as an exotic neutral excitation in superconductors, is the building block of topological quantum computing. It has recently been found in the vortices of several iron-based superconductors as a zero-bias conductance peak (ZBCP) in tunneling spectroscopy. In particular, a clean and robust MZM has been observed in the cores of free vortices in (Li0.84Fe0.16)OHFeSe. Here using scanning tunneling spectroscopy (STS), we demonstrate that Majorana-induced resonant Andreev reflection occurs between the STM tip and this zero-bias bound state, and consequently, the conductance at zero bias is quantized as 2e 2 /h. Our results present a hallmark signature of the MZM in the vortex of an intrinsic topological superconductor, together with its intriguing behavior.A peak at zero energy in tunneling spectroscopy is an important hallmark, but not sufficient proof, for identifying a MZM. For example, although the existence of a MZM is predicted at the ends of a strongly spin-orbital-coupled semiconductor nanowire in the presence of a proximate superconductor and a large Zeeman field [1-7], there are alternative interpretations for the experimental observations of zero energy peaks in these systems [8][9][10][11][12]. Morecompelling evidence for a MZM is that the zero-bias peak possesses the quantized universal conductance, G0 = 2e 2 /h, in tunneling experiments, due to resonant Andreev reflection and the particle-hole symmetry of the MZM [13]. At zero temperature, this Majorana-induced resonant Andreev reflection (MIRAR) would give the quantized conductance regardless of the coupling strength [13]; and at finite temperature, the quantized conductance may be observed when the tunneling coupling is sufficiently strong [1,14]. The quantized conductance of a ZBCP was first observed in the tunneling spectrum of a hybrid device between superconducting aluminum and an InSb nanowire [15]. It thus strongly supports the existence of MZMs in semiconductor nanowire devices, although the ultimate proof of Majorana physics would be a demonstration of non-Abelian statistics [16][17][18]. *