  We report the design and performance of the hybrid electromagnetic
calorimeter consisting of 1152 $PbWO_4$ crystals and 576 lead glass blocks for
the PrimEx experiment at the Jefferson Laboratory. The detector was built for
high precision measurement of the neutral pion lifetime via the Primakoff
effect. Calorimeter installation and commissioning was completed with the first
physics run in fall of 2004. We present the energy and position resolution of
the calorimeter. Obtained $\pi^0$ mass resolution of $1.3 \mathrm{MeV/c^2}$ and
its production angle resolution of $0.34 \mathrm{mrad}$ demonstrate the ability
of the experiment to extract the $\pi^0$ lifetime on one percent level.
