Ablation of aluminum target by multipl e femtosecond laser pulses is investigated by recording the time-resolved shado wgraphs of the dynamic ablation process.The scanning electron microscopic (SEM) measurements of the ablation sites are performed to analyze the surface morphology and its in fluence on the ablation process.With the help of shadowgraphs recorded at different time delays,it is fo und that the spatial distribution of the ejected material and the expansion of the shock wave generated during the ablation vary noticeably with the number of laser pulses.For the initial one or two pulses,nearly concentric a nd semicircular stripes within the shock wave front are observed at 1ns delay time.For subsequent pulses,howev er,such stripes become blurred and almost invisible.The SEM images of the ablation sites reveal some m icro- and nano-structures formed by condensed aluminum droplets and molten streams generated through laser ablation.Ablation by multiple femtosecond pulses exhibits different characteristics compared with the case by a single femtosecond pulse,which is primarily due to the surface morphology change induced by the pr eceding pulses on the target surface.These results should provide a meaningful guideline in applications of femtosecond laser such as pulsed laser deposition and micro-machining by direct laser writing.