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In this paper, a novel decoupling method without adding any decoupling structure is investigated, and it is able to apply to multiple-input and multiple-output (MIMO) antenna designs with dual and multiple elements. The decoupling mechanism applied in here is to change the electrical length of the radiators to manipulate the radiation pattern for obtaining pattern diversity and good isolation simultaneously. The design mechanism and procedures for realizing pattern diversity are presented. To validate the proposed method, a two-element monopole antenna array with inverted-L elements is initially studied with the isolation better than 20 dB. Next, a four-element MIMO antenna utilizing two different types of monopole elements is proposed to further verify the feasibility of the proposed decoupling method. Finally, an eight-element MIMO antenna operating at 3.5 GHz is implemented with a compact total size of $40\times 100$ mm2, and the measured isolations between two elements are better than 15 dB. Envelope correlation coefficient and channel capacity are calculated to evaluate the MIMO performance, and good results are obtained. The target application of the proposed design is for laptop, small cell base station, or WiFi box with multiple elements. [ABSTRACT FROM AUTHOR]