We computed a grid of theoretical models to fit the delta Scuti frequencies of CoRoT 100866999 detected earlier from the CoRoT time serials. The pulsating primary star is determined to be a main-sequence star with a rotation period of 4.1(-0.5)(+0.6) days, rotating slower than the orbital motion. The fundamental parameters of the primary star are determined to be M = 1.7(-0.04)(+0.13) M-circle dot, Z = 0.012(-0.000)(+0.004), f(ov) = 0.02(-0.02)(+0.00), T-eff = 8024(-297)(+249) K, L = 11.898(-1.847)(+2.156) L-circle dot, log g = 4.166(-0.002)(+0.013), R = 1.787(-0.016)(+0.011), R-circle dot, and X-c = 0.488(-0.020)(+0.011), matching 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), are suggested 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-conv/R = 0.0931(-0.0013)(+0.0003).