The gravitational radiation capture between unequal mass black holes without spins has been investigated with numerical relativistic simulations, and the results are compared with the Post-Newtonian (PN) approximations. We adopt the parabolic approximation which assumes that the gravitational wave radiation from a weakly hyperbolic orbit is the same as that from the parabolic orbit having the same pericenter distance. Using the radiated energies from the parabolic orbit simulations, we have obtained the impact parameters ($b$) of the gravitational radiation captures for weakly hyperbolic orbits as a function of initial energy. The most energetic encounters occur around the boundary between the direct merging and the fly-by orbits, and could emit several percent of the total ADM initial energy even from such fly-by orbits. Energy and angular momentum radiated in the case of unequal mass black holes are smaller than those of equal mass black holes at the same initial orbital angular momentum provided that their total masses are the same. We show that the agreement for impact parameters between the PN approximation and the numerical relativity begins to break down as encounter becomes very close ($\it{e.g.}$, $b \lesssim 100$ M), and that disagreement becomes more conspicuous at higher mass ratios. For instance, the impact parameters could differ over $50\%$ from those in the PN approximation if the impact parameter is $b\lesssim30$ M for the mass ratio of $m_{1}/m_{2}=16$. ; Comment: 9 pages, 10 figures