We quantitatively investigate intensity fluctuations observed with the Yohkoh SXT, which is sensitive to hot (>2 MK) plasma, and TRACE, which is sensitive to cool (～1 MK) plasma. We find that the TRACE light curves contain fluctuations that are significantly larger than the photon noise and that TRACE is more sensitive to the emission from nanoflare heating than is the SXT. We discover that the standard deviation of the fluctuation (the photon noise is removed) is proportional to the mean intensity for both the SXT and TRACE loops. We also analyze the autocorrelation functions in order to obtain the duration of the intensity fluctuations. While the duration of the intensity fluctuations for the SXT loops is relatively short because of the significant photon noise, that for the TRACE loops agrees well with the characteristic cooling timescale. This is evidence that coronal loops are continuously heated by impulsive nanoflares. We estimate the energy of nanoflares to be 10~(25) ergs for SXT loops and 10~(23) ergs for TRACE loops. The occurrence rate of nanoflares is about 0.4 and 30 nanoflares s~(-1) in a typical SXT loop and a typical TRACE loop, respectively.
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