In this paper, we develop a novel robust scheme of two-dimensional unequal error protection (2-D UEP) for the H.264 scalable video coding (SVC) with a combined temporal and quality (SNR) scalability over packet-erasure channel. To avoid the waste of bits and obtain the best rate allocation, we propose a threshold-based UEP (TH-UEP) scheme. The proposed TH-UEP designs a predefined threshold according to the length of packet and the error correcting ability of RS code to achieve the best allocation. In addition, the proposed scheme also derives a simple mathematical model to reduce computational load of the best allocation. Experimental results demonstrate that the proposed H.264 video transmission scheme can provide strong robustness and video quality improvement when compared to other 2-D UEP schemes.