An electronic textile and wearable electronic have attractive much attention, because it is effective for the sensors to be close in contact to the subject in order to measure biometric information with precision. Thus, stretchable wires that are compatible with fabric substrate are intensively studied. In particular, printable elastic conductors are expected to play an important role to fabricate large-area stretchable sensor/actuator networks on fabric and elastomeric substrates. In this presentation, we report recent progress of printable elastic conductor for smart apparel and wearable electronics applications. Our elastic conductors are fabricated by mixing Ag flakes with the typical sizes of a few micrometers, fluorine rubbers, and surfactant. Containing. We found that Ag nanoparticles are formed in situ after printing and annealing processes. Our printable elastic composites exhibit conductivity of 935 S cm-1 when stretched up to 400%. Ag nanoparticle formation, which is the key to achieve a drastic improvement of conductivity, is controlled by the surfactant, heating processes, and elastomer molecular weight. Fully printed sensor networks with elastic conductors are demonstrated for e-textile and wearable electronic applications.