  We computed a grid of theoretical models to fit the $\delta$ Scuti
frequencies of CoRoT 100866999 detected earlier from the CoRoT timeserials. The
pulsating primary star is determined to be a main sequence star with a rotation
period of $4.1^{+0.6}_{-0.5}$ days, rotating slower than the orbital motion.
The fundamental parameters of the primary star are determined to be $M$ =
$1.71^{+0.13}_{-0.04}$ $M_{\odot}$, $Z=0.012^{+0.004}_{-0.000}$, $f_{\rm ov}$ =
$0.02^{+0.00}_{-0.02}$, $T_{\rm eff}$ = $8024^{+249}_{-297}$ K, $L$ =
$11.898^{+2.156}_{-1.847}$ $L_{\odot}$, $\log g$ = $4.166^{+0.013}_{-0.002}$,
$R$ = $1.787^{+0.040}_{-0.016}$ $R_{\odot}$, and $X_{\rm c}$ =
0.488$^{+0.011}_{-0.020}$, matching well those obtained from the eclipsing
light curve analysis. Based on the model fittings, $p_1$ and $p_5$ are
suggested to be two dipole modes, and $p_3$, $p_4$, $p_6$, and $p_7$ to be four
quadrupole modes. In particular, $p_4$ and $p_7$ are identified as two
components of one quintuplet. Based on the best-fitting model, we find that
$p_1$ is a g mode and the other nonradial modes have pronounced mixed
characters, which give strong constraints on the convective core. Finally, the
relative size of the convective core of CoRoT 100866999 is determined to
$R_{\rm conv}/R$ = $0.0931^{+0.0003}_{-0.0013}$.
