A study of the substructure of jets with transverse momentum
greater than 400 GeV/c produced in proton-antiproton collisions at a
center-of-mass energy of 1.96 TeV at the Fermilab Tevatron Collider and
recorded by the CDF II detector is presented. The distributions of the
jet mass, angularity, and planar flow are measured for the first time
in a sample with an integrated luminosity of 5.95 fb-1. The observed
substructure for high mass jets is consistent with predictions from
perturbative quantum chromodynamics.