The poly(4-methyl-l-pentene) sample was used to investigate the cavitation-induced stress-whitening phenomenon during stretching at different temperatures via the ultrasmall-angle X-ray scattering technique. Two modes of cavitation were found that mode I cavitation activated around yield point followed by mode II cavitation generated in highly oriented state. The critical strain for initiating the mode H cavitation increases with the increase of the stretching temperature, whereas the critical stress grew steadily in the lower temperature regime (30-60 degrees C) and reached a plateau at 70 degrees C. The appearance of mode H cavitation at large strains was independent of the mode I cavitation. The mode I cavitation was attributed to the competitive process between the formation of cavities and shearing yield of lamellae, whereas the mode II cavitation was proven to be related to the failure of the whole highly oriented entangled amorphous network because of the breaking of interfibrillar load -bearing tie molecules. Size distribution of cavities has been successfully calculated using a model fitting procedure. The results showed that the quantity of cavities increased heavily while the size was kept nearly constant during the propagation of the mode II cavitation.