In this study, we try to fabricate magnetic tunnel junctions with organic spacer. Two processing techniques, Langmuir-Blodgett (LB) method and thermal evaporation, are applied for the preparation of the organic layer. While the stearic acid (SA) and BEDT-TTF (ET) are deposited by LB method, pentacene (Pn) film is prepared by thermal evaporation system. In the first part of this thesis, we grow ET and SA by LB film method. With the help of four-terminal measurement, the measured junction resistance (Rj) is positive as the spacer is four mono-layer (ML) and negative if the spacer is less than three ML. The negative measured resistance is a result of the smaller junctions’ resistance as compared to the resistance of electrodes. In addition, magnetoresistance (MR) effect can not be observed in these junctions. In order to increase Rj, Al2O3 of 1 nm thickness is deposited prior to the subsequent growth of LB films. By introducing the Al2O3 to the junctions, Rj may increase to the range of 102 ohm and 106 ohm, while the LB-prepared spacer are one and two ML, respectively. In the second part, the Pn spacer layer is deposited by thermal evaporation. However, the Rj is negative and there is no MR effect. Therefore, we intentionally exposed Pn to oxygen plasma in order to remodel the Pn films. As a result, the Rj increases and the two-step R-H loop is found to consist with M-H loop. Hence, we tried the fine tunings of preparation conditions, such as the exposure time in oxygen plasma, thickness of Pn and annealing temperature. MR effect can then be observed. Moreover, the MR ratio may be up to 0.5 %, if Pn films of thickness 14 nm are annealed together with CoFe layer at 130 ℃ and 157 ℃ for 1 hour and exposed to oxygen plasma for 4 minutes at room temperature. The change of Pn to “6, 13-pentacenequinone”, an endoperoxide or the formation of peroxy bridge between Pn molecules are thought responsible for the results.