The 3D spontaneous fast reconnection model is applied to well-known signatures of geomagnetic substorms and solar flares. First, it is applied to the traveling compression regions (TCRs) associated with plasmoids propagating down the tail plasma sheet, known its a definite signature of geomagnetic substorms. and the in-situ satellite observations can be precisely explained, both qualitatively and quantitatively. Then. it is demonstrated that the magnetospheric current wedge to link the fail Current,e drastically evolves through field-aligned currents to the auroral electrojet. It is also found that the well-known morphological features of two-ribbon flares call be explained by the fast reconnection model. In particular. the joule heating, associated with the flare Current wed-e. is shown to be important for the two-ribbon heating. Therefore, it is suggested that both solar flares and geomagnetic substorms result from the same physical mechanism, i.e., the fast reconnection mechanism.