We propose a novel method of multilevel in-memory-searching (IMS) using 3D NAND-flash memory. The approach provides ultra-high parallel searching capability with the database stored in high-density 3D NAND-flash IMS chip(s), with the power similar to a typical NAND flash read operation. Fail bit issue caused from retention loss, program/read disturbance are evaluated and solutions are provided. An example on applying the MLC NAND-IMS accelerator in deoxyribonucleic acid (DNA) read mapping is described. The multilevel IMS is promising for data-intensive applications including big data searching and future memory-centric computing systems.