Vaporization and liquid metal expulsion are the two main mechanisms of material loss during laser microjoining. Various factors that affect alloying element vaporization and liquid metal expulsion during microjoining of 304 stainless steel with short Nd-yttrium aluminum garnet (YAG) laser pulses were investigated experimentally and theoretically. The temperature fields used to simulate the loss of materials were obtained from a well-tested comprehensive three-dimensional transient heat transfer and fluid flow model. The calculated fusion zone geometry and composition change produced by laser microjoining under various welding conditions showed fair agreement with the corresponding experimental results. The conditions necessary for the initiation of liquid metal expulsion were determined by balancing the vapor recoil force with the surface tension force at the periphery of the liquid pool. The laser power density and pulse duration are important parameters for liquid metal expulsion. Higher power density and longer pulse duration increases the tendency of liquid metal expulsion during laser microjoining.