In this experimental study, flow boiling in mini/microtubes was investigated with surface enhancements provided by crosslinked polyhydroxyethylmethacrylate (pHEMA) coatings, which were used as a crosslinker coating type with different thicknesses (∼50 nm, 100 nm, and 150 nm) on inner microtube walls. Flow boiling heat transfer experiments were conducted on microtubes (with inner diameters of 249 μm, 507 μm, and 908 μm) coated with crosslinked pHEMA coatings. pHEMA nanofilms were deposited with initiated chemical vapor deposition (iCVD) technique. De-ionized water was utilized as the working fluid in this study. Experimental results obtained from coated microtubes were compared to their plain surface counterparts at two different mass fluxes (5000 kg/m2 s and 20,000 kg/m2 s), and significant enhancements in critical heat flux (up to 29.7%) and boiling heat transfer (up to 126.2%) were attained. The enhancement of boiling heat transfer was attributed to the increase in nucleation site density and incidence of bubbles departing from surface due to porous structure of crosslinked pHEMA coatings. The underlying mechanism was explained with suction-evaporation mode. Moreover, thicker pHEMA coatings resulted in larger enhancements in both CHF and boiling heat transfer.