Flow boiling in microchannel is a promising way of high heat flux removal. In account of this, numerous experimental and numerical studies have been performed on flow boiling in conventional as well as in microchannels focusing on heat transfer, bubble dynamics and flow pattern while studies on conjugate heat transfer in flow boiling are yet to be explored. This work presents a systematic three-dimensional numerical study of conjugate effect of heat transfer in two-phase flow in a microtube subjected to constant wall heat flux. Volume of fluid (VOF) model along with phase change model are used to capture bubble shape and flow pattern. Water enters the microtube at a temperature of 300 K. The effect of (i) wall to fluid conductivity ratio (ksf), (ii) wall thickness to inner radius ratio (δsf), and (iii) inlet mass flux (G) in unsteady flow boiling condition are explored. Transformation of nucleating bubbles into elongated bubbles are characterised in terms of wall thickness (δsf) and its material conductivity (ksf). The results indicate that bubble length increases with increasing wall material conductivity but decreases with increasing wall thickness as well as mass flux. Growth and motion of vapor bubbles due to phase change are also explored. Comparison between single and two-phase flow under conjugate heat transfer scenario is also discussed. © 2019