The present invention relates to a method and an apparatus for estimating a clock skew. The method comprises: obtaining a basic clock skew of each clock tree in the circuit; judging whether two units are in a same clock domain; if they are in different clock domains, estimating the clock skew between units to be a larger one of basic clock skews of the clock trees corresponding to these two unit; if these two units are in the same clock domain, further judging whether they are in a same hierarchical logic block; if they are in different hierarchical logic blocks, estimating the clock skew between units to be the basic clock skew of the clock tree of these two units plus additional clock skew caused by different hierarchical logic blocks. The apparatus is corresponding to the method. With the method and apparatus, the clock skew in the circuit can be estimated more accurately, which improves the efficiency of circuit design.