A modular block wall system is comprised of a plurality of differently sized blocks. Each block comprises a front surface and opposing rear surface, a top surface and opposing bottom surface, and first and second opposing tapered side surfaces. Blocks further includes a recess extending inwardly from top surface and a protrusion extending outwardly from bottom surface. Blocks may also include a plurality of cores through the block from top surface to bottom surface. Blocks may further include a removable portion that can be cleaved off to create a decorative face on rear surface. The projection on each block is configured to fit within the recess of a block in the next lower course of blocks, regardless of the sizes of the respective blocks. Preferably, the varying lengths and heights of the blocks are evenly divisible by a uniform dimension. This allows modular blocks to be easily put together in any configuration to create a finished looking, yet non-uniform wall.