In August 17, 2017, the LIGO-Virgo scientific collaboration, for the first time, detected a gravitational wave (GW) signal from a binary neutron star (NS) merger event. Multi-wavelength electro-magnetic (EM) counterparts to GW170817 have also been detected. This has opened up a new era of GW astronomy, whereby comprehensive analyses of multi-messenger signals will shed light on both GW and EM studies. For NS-NS mergers, many associated EM counterparts have already been proposed in the literature, and their relative brightness is essentially determined by the properties of the merger remnant. In this talk, I will briefly review the possible EM signatures for different merger remnants. Moreover, I will talk about how to use the multi-messenger observations for NS-NS mergers to make constraints on the maximum mass of NS.