It is the current trend that microprocessors extend their instruction set to speedup the multimedia applications. Intel'sMMX, SSE and SSE2 are good example. But do the multimediaapplications favorthesearchitecturefeatures? And does thecurrentsoftwaresupporttheseinstruction sets and make full use of them? These questions are of muchconcerntothe researchers. Weinvestigate theperformancecharacteristics of 2MPEG4Videoplayers on an Intel P4 baseddesktop PC bymeans ofdynamicprofiling.BasedonIntelVtune, a hardwareperformancemonitoring tool, we obtainaccurateperformancedata. And we make indepthanalysis of the obtained data based on themachine'sarchitectureparameters andperformanceprofiles. We find that the 2applicationsmakegood use of 64-bit MMXinstructions. But theyrarely use SSE and SSE2instructions. Theperformancedegradation due to Cachemisses at tow levels of Caches is remarkable. Toenhance theperformance ofMPEG4 mediaapplications,improvingmemoryhierarchyperformance andoptimizing themediaplayers are moreeffective ways thanemploying moreadvancedmultimediainstruction sets.