Among the RHESSI flare samples,we concentrated on a kind of flare that presents two successive peaks(that is,it presents both an impulsive phase and a gradual phase)in 12-25 keV light curves.Taking the C1.4 flare on 2002 August 12 as an example,we studied the light curves,spectra,and images in detail.Making full use of the capabilities of RHESSI,we showed some evidence to support the expected causal relationship between these two peaks:the first peak is mainly nonthermal,while the second peak is mainly thermal;the energy carried by nonthermal electrons during the first peak seems to be comparable to the thermal energy of the second peak.The morphologies of X-ray images and their evolutions provide additional evidence for this causality.We conclude that two such peaks in the 12 - 25 keV light curve are good evidence for the chromospheric evaporation.However,the maximum time of the second peak is later than the end time of the first peak,suggesting that for some events,a modification of the traditional Neupert effect could be necessary by inclusion of a time delay,which might be partly related to the filling of the loop by evaporated material.