The swallow-tailed defect is a typical defect in MBE HgCdTe which has a uniform and regular shape. The morphology， structure and growth mechanism of swallow-tailed defect were investigated. Two raised swallow-tails are major characteristic on the surface， and an inverted pyramid structure surrounded by (11ˉ1ˉ)， 1ˉ1ˉ1， (11ˉ1)， (111ˉ) and （211） crystal faces is demonstrated in the film of the defect. The swallow-tailed defect is （552）A twin defect， the difference of growth rate between （552）A twin crystal and （211）A matrix is the root cause of defect formation. The nucleation sites and growth planes of （552）A twin crystal， 1ˉ1ˉ1 and (11ˉ1) crystal plane， which determines the morphology and structure of swallow-tailed defect， depend on different Schmid factor of 12 slip systems in HgCdTe.