Heat flow measurements have become essential in all sectors of industry as energy, chemicals, automotive, aerospace, environment, medicine and food industry, etc... On one hand, it allows to quantify heat losses through a part of a system surface, on the other hand it also permits to get some information from inside the studied system. Moreover, these measurements allow to predict or follow in real time the system fluctuations to external thermal disturbances and this, well before any temperature sensor would have detected the lesser change. However, these heat flow measurements are very delicate and difficult/harsh to implement. Metrological quality needs heat fluxmeters (ie heat flux sensors, HFs) that diplay accuracy, stability and that are not intrusive. Generally, such devices are built up onto polymer substrates1. In this context, we have developed an original heat flux microsensor (μHFS) based on Si technology, which has a high sensitivity and a low thermal resistance. It relies on the use of mesoporous Silicon boxes locally etched into a Si-(100) wafer by anodic chemical, method, such a way to make thermal dissymetries2. This Si anodisation allows reducing drastically the Silicon thermal conductivity from 140 W/m/K to some 1W/m/K in the porous areas (boxes). The sensitive component of the μHFS is a planar Gold/polySilicon based thermopile made up with a large number of plated thermocouples associated in series (up to 500 cells) distributed on the periodical distribution of Si-porous boxes. This kind of devices present responsivities ranging up to 260 mV/W with electrical resistance lower than a hundred kW. This presentation will describe the fabrication process of these sensors and their operating principle and the main results will be discussed.