The invention provides a process and kit for isolating and purifying nucleic acids such as DNA or RNA or a hybrid molecule of DNA and RNA. A siliceous material combined with a solution is used to prepare highly pure nucleic acids, especially DNA. The siliceous material is a support for absorbing target materials, and the solution according to this invention promotes the target materials to bind to the siliceous material, especially to promote DNA to bind to the siliceous material. The solution is an acid and potassium ion-containing aqueous solution. The invention further provides DNA prepared by using the process. Because no chaotropic agents or other poisonous or costly agents are used in the process, DNA prepared by the process may be used widely, especially in food industry and pharmaceutical industry.