Based on X-ray mineral identification, particle-size analysis, electron microscopic identification and field observation of purple soil and purple mudstone, mineral composition and particle size composition as well as susceptibility to erosion were studied. Results show that little difference was observed in mineral composition and content between purple soil and purple mudstone, which are both mainly composed of quartz and feldspar, plus Smectite-dominated clay mineral. They are all very close in particle size and consisted dominantly of fine silt and clay. The micro-structure of purple soil appears to be mainly of clasts with pores cemented, and some of granulars cemented through contact and of granular cemented with lumps. Clay played a key role in cementing, of which only about 3% was attributed to Fe₂O₃, chemical agglutinate. The microstructure of purple mudstone is characterized predominantly by miniature laminate structure and micro-lump structure, which take up the main part. The high smectite content indicates that both purple soil and purple mudstone are dilatable soils. Purple soil and purple mudstone frequently swell and shrink when the weather is humid, making them susceptible to physical weathering and erosion and their miniature block structure, miniature laminate structure and dense micro-pores are also important causes of its poor erosion resistance. These structures of the purple mudstone are mainly formed during the diagenetic process, as a result of the compression during the diagenetic process of the large amounts of spheric and ellipsoidal clays formed during primitive sedimentation.