The advance in nanotechnology has enabled the fabrication of nanomachines for health applications. Recently, molecular communication has become a promising communication paradigm that allows nanomachines to exchange information by using messenger molecules in fluid environments. To enable molecular communications, the knowledge of distance between nanomachines is critical since the distance affects both the performance and the efficiency of molecular communication. However, works on molecular communication either assume the distance is known or the distance estimation is based on the assumption of clock synchronization between nanomachines. In this paper, we propose novel methods for distance estimation using only one-way transmission and requiring no clock synchronization between nanomachines. The noise of diffusion channel due to random walk of molecules is investigated and methods to effectively improve the estimation accuracy are proposed.