Coherent radiation near 125 nm is generated by 61S0-71S0 two-photon resonant four-wave sum mixing (ω4= 2ω1+ω3) in Hg vapor. The two-photon resonant enhancement and two-photon resonant Kerr effect have been investigated. The optimum phase-matching conditions of plane wave and focusing beam have been analysed. The signals of four-wave parametric oscillation and four-wave difference frequency mixing have also been observed.