Lanthanide phosphates are of increasing interest because of their sharp, stable photoluminescence with long lifetime. The emission of lanthanide phosphate can be easily changed by introducing different lanthanide ion dopants such as Eu and Tb for down-conversion and Er, Tm for up-conversion luminescence. However, the synthesis of such crystalline materials is always energy- and/or time-consuming. Sometimes, even templates and/or organic solvents are required to prepare the nano-sized and crystalline lanthanide phosphate. Those obstacles hinder the scaling-up of production and practical applications. In this communication, we report two microfluidic platforms (SDP and RTP) using continuous flow conditions to fabricate (within seconds) ultrathin single-crystalline rhabdophane lanthanide phosphate nanowires with tunable length at room temperature, using water as a benign medium.