We perform 3D numerical simulations for merger of equal mass binary neutron stars in full general relativity preparing irrotational binary neutron stars in a quasiequilibrium state as initial conditions .
Simulations have been carried out for a wide range of stiffness of equations of state and compactness of neutron stars , paying particular attention to the final products and gravitational waves .
We take a fixed uniform grid in Cartesian coordinates with typical grid size ( 293 , 293 , 147 ) in ( x,y,z ) assuming a plane symmetry with respect to the equatorial plane .
A result of one new large-scale simulation performed with grid size ( 505 , 505 , 253 ) is also presented .

We find that the final product depends sensitively on the initial compactness of the neutron stars : In a merger between sufficiently compact neutron stars , a black hole is formed in a dynamical timescale .
As the compactness is decreased , the formation timescale becomes longer and longer .
For less compact cases , a differentially rotating massive neutron star is formed instead of a black hole .
In the case of black hole formation , the disk mass around the black hole appears to be smaller than 1 \% of the total rest mass .
It is also indicated that waveforms of high-frequency gravitational waves after merger depend strongly on the compactness of neutron stars .