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In this note, we consider a holomorphic mapping f from the unit disk D in C to p-ball Bp={z ∈Cn:∑ni=1|zi|p<1},1<p<+∞. It is proved that for such f,|▽||f||(z)|≤1-||f(z)||2/1-|z|2,z∈D.The extremal problem is also discussed when p is an even number.