Cloud-cloud collision has been regarded as one of the important mechanisms which triggers star formation. But a detailed analysis on the change of the critical mass has been absent due to the poor resolution of numerical experiments or limited by the assumption of a geometrical symmetry. We simulate the collisions between isothermal interstellar clouds using a three-dimensional hydrodynamic code (Smoothed Particle Method). The simulation in three dimensions gives us not only the criterion for the trigger of star formation but also the information about the origin of the interstellar cloud rotation and its initial distribution of angular momentum.