The disclosed methods in the application relates to making an adapter-tagged restriction library. The library or libraries created with the disclosed methods avoid the self-ligation of the restriction fragments. The application further disclosed methods of using the library or libraries to measure the methylation level of a genome, or comparing methylation levels among two or more genomes. The measurement of the global methylation levels can be achieved with quantitative PCR method by measuring the number of restriction fragments in the libraries. It also can be used for next generation Sequencing (NGS), Copy Number Variation (CNV), restriction site mutation, endogenous gene jumping, and exogenous DNA insertion, somatic hypermutation, gene knockout/knock in etc.