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.