  The Young's modulus of graphene is investigated through the intrinsic thermal
vibration in graphene which is `observed' by molecular dynamics, and the
results agree quite well with the recent experiment [Science \textbf{321}, 385
(2008)]. This method is further applied to show that the Young's modulus of
graphene: 1. increases with increasing size and saturation is reached after a
threshold value of the size; 2. increases from 0.95 TPa to 1.1 TPa as
temperature increases in the region [100, 500]K; 3. is insensitive to the
isotopic disorder in the low disorder region ($< 5%$), and decreases gradually
after further increasing the disorder percentage.
