A fast intra skip detection algorithm based on the ratedistortion (RD) cost for an inter frame (P-slices) is proposed for H.264/AVC video encoding. In the H.264/AVC coding standard, a robust rate-distortion optimization technique is used to select the best coding mode and reference frame for each macroblock (MB). There are three types of intra predictions according to profiles. These are $16{ imes}16$ and $4{ imes}4$ intra predictions for luminance and an $8{ imes}8$ intra prediction for chroma. For the high profile, an $8{ imes}8$ intra prediction has been added for luminance. The $4{ imes}4$ prediction mode has 9 prediction directions with 4 directions for $16{ imes}16$ and $8{ imes}8$ luma, and $8{ imes}8$ chrominance. In addition to the inter mode search procedure, an intra mode search causes a significant increase in the complexity and computational load for an inter frame. To reduce the computational load of the intra mode search at the inter frame, the RD costs of the neighborhood MBs for the current MB are used and we propose an adaptive thresholding scheme for the intra skip extraction. We verified the performance of the proposed scheme through comparative analysis of experimental results using joint model reference software. The overall encoding time was reduced up to 32% for the IPPP sequence type and 35% for the IBBPBBP sequence type.