The cirrus clouds tightly connected with tropical deep convective clouds can extend and persist for some hours after the deep convective clouds themselves dissipate. This can result in time lags of the diurnal cycle of deep convective clouds detected from infrared satellite measurements with different brightness temperature thresholds because different amounts of cirrus clouds contaminate the measurement. The diurnal cycles of rain from the Tropical Ocean--Global Atmosphere (TOGA) radar during the Tropical Rainfall Measuring Mission (TRMM) Wet Season Atmospheric Mesoscale Campaign (WETAMC) Large-Scale Biosphere Atmosphere (LBA) Experiment in Amazonia and the diurnal cycles of deep convective clouds and high cold clouds from the Precipitation Radar (PR), Visible and Infrared Scanner (VIRS) onboard the TRMM satellite over the tropics (30¿S--30¿N) from November 1998 to April 1999 are investigated to study the influence of cirrus clouds on the observed diurnal cycle of tropical deep convective clouds. A 2-hour time lag of the diurnal cycle of deep convective clouds from the VIRS with respect to that from the PR is found over land. Over ocean the cirrus clouds generated by deep convective clouds enhance the diurnal cycle of the deep convective clouds from the VIRS, and a time lag similar to that over land also occurs. The influence of cirrus clouds leads the diurnal cycle of the deep convective clouds from the VIRS to depend strongly on the selected IR threshold and to be very different from that of the PR over the maritime continent. Moreover, over ocean and the maritime continent, from late afternoon to midnight the strong increase of the deep convective clouds from the VIRS is mainly due to the developing cirrus clouds near and above the tropical tropopause layer.