  We discuss a supersymmetric version of DBI (Dirac-Born-Infeld) inflation,
which is a typical inflation model in string cosmology. The supersymmetric DBI
action together with a superpotential always leads to correction terms
associated with the potential into the kinetic term, which drastically change
the dynamics of DBI inflation. We find two significant features of
supersymmetric DBI inflation. The first one is that ultra-relativistic motion
is prohibited to cause inflation, which leads to order of unity sound velocity
squared and hence small non-Gaussianities of primordial curvature
perturbations. The second one is that the relation between the tensor-to-scalar
ratio and the field variation is modified. Then, significant tensor-to-scalar
ratio $r \gtrsim 0.01$ is possible even for sub-Planck variation of the field.
These new features are in sharp contrast with those of the standard
non-supersymmetric DBI inflation and hence have a lot of interest implications
on upcoming observations of cosmic microwave background (CMB) anisotropies by
the Planck satellite as well as direct detection experiments of gravitational
waves like DECIGO and BBO.
