Site amplification effects of Aranayake landslide area in Sri Lanka are analyzed using records observed by a wide dynamic-range earthquake motion array with soil- and rock-site stations. We find that the rock site on the mountain ridge does not have the same response as the rock site at the foot of mountain, which has a characteristic frequency-dependent site effect. The relative amplification of the mountain ridge rock site to the mountain foot rock site is nearly 1.0 at frequencies of 2 < f < 6 Hz, having a peak of 2 or more at 6 to 8 Hz, that gradually decreases with increasing frequency, and becomes less than 1 above 10 Hz. On the land slide slope in Aranayake, clear resonant vibration with a frequency of nearly 12 Hz and an amplification of about 20 times is found from the amplification analysis of the main part of the S-wave. The vibration is excited immediately after P-wave arrival and continues to S-wave coda. Particle motions of a 12-Hz component in main portion of the S-wave show that the vibration is almost horizontal, and that its predominant direction nearly matches the landslide axis, but gradually changes with time according to the direction of stress free. From the survey of seismic activity in Aranayake landslide area, it is determined that microearthquakes with Ts-p < 1.0 s and the velocity magnitude 2 < M < 3 occur several ten times a month. The landslide slope is frequently attacked by such microearthquakes, and is shaken by the 12-Hz resonant vibration.