Electroactive　species　modified　electrodes　are　extensively　studied　as　the　mediator　donor　in　homogeneous　electrocatalytic　reaction　for　electrochemical　sensing　purpose.　In　this　paper,　homogeneous　electrocatalytic　reaction　within　a　thin　layer　modified　electrode　is　investigated　theoretically　and　experimentally　to　illustrate　the　electroactive　species　distribution　on　ferrocenedicarboxylic　acid　modified　carbon　black　paste　electrodes　(FDACBPE).　The　voltammetric　analysis　of　the　fabricated　FDACBPE　reveals　that　the　electrode　is　in　diffusional　control　at　the　scan　rate　lower　than　50　mV　s-1　while　a　surface　control　electrode　reaction　is　observed　at　a　higher　scan　rate.　Based　on　the　experimental　results,　three　finite　element　models　including　thin　layer　model,　fixed　mediator　model　and　combined　model　are　established　to　evaluate　the　homogeneous　electrocatalytic　reaction　with　FDACBPE.　The　numerical　results　show　that　the　anodic　peak　gap　order　of　3　models　is　as　follows:　fixed　mediator　model　＞　combined　model　＞　thin　layer　model.　As　validation　of　the　proposed　model,　the　comparison　between　experimental　results　and　numerical　data　on　both　polished　and　unpolished　FDACBPE　indicates　that　the　combined　model　data　are　more　suitable　for　the　description　of　experimental　voltammograms　compared　with　the　other　two　models.　Also,　the　comparison　between　these　models　suggests　that　both　surface　confined　and　diffusional　FDA　exist　when　FDACBPE　is　employed.