The study of incompressible magnetohydrodynamic (MHD) turbulence gives useful insights on many astrophysical problems. We describe a pseudo-spectral MHD code suitable for the study of incompressible turbulence. We review our recent' works on direct three-dimensional numerical simulations for MHD turbulence in a periodic box. In those works, we use a pseudo-spectral code to solve the incompressible MHD equations. We first discuss the structure and properties of turbulence as functions of scale. The results are consistent with the scaling law recently proposed by Goldreich & Sridhar. The scaling law is based on the concept of scale-dependent isotropy: smaller eddies are more elongated than larger ones along magnetic field lines. This scaling law substantially changes our views on MHD turbulence. For example, as noted by Lazarian & Vishniac, the scaling law can provide a fast reconnection rate. We further discuss how the study of incompressible MHD turbulence can help us to understand physical processes in interstellar medium (ISM) by considering imbalanced cascade and viscous damped turbulence.