Mode of $CaCl_2$ mediated transformation in E. coli was studied with using PBR322 DNA and Charon4 DNA. $CaCl_2$ and heat shock was essential for transformation of E. coli. Optimal transformation occurred in 50 mM $CaCl_2$ and for 2 min of heat shock at 42$^\circ$C. Transformation occurred by heat shock with $CaCl_2$ but further incubation in ice for 30 min increased the transformation frequency. E. coli cells that heatshocked in $CaCl_2$ were transformable with foregin DNA and heat-shock of the mixture (donor DNA and recipient cells) was not essential. But heat-shock concerned in not only inducing competency but also uptake of foreign DNA. $CaCl_2$ had an effect on removal of transformation barrier of E. coli cells, rather than conformational change of donor DNA.