•Resistivity monitoring of two types Co-rich microwires during DC Joule heating treatment.•Microwire temperature definition during Joule heating, using a crystallized microwire.•Cr-doped microwires had well-marked resistivity minimum near the temperature 163 °C.•The resistivity minimum shifted to a temperature value 260 °C after annealing.•Below the temperature minimum, resistivity dependence is proportional to -ln(T). The paper investigates the electrical resistivity of two types of the glass-coated Co-rich amorphous ferromagnetic microwires during the Joule heating by direct current. We measured the relationships between the microwire’s resistivity and the power, which applied to the samples in as-prepared and crystallized conditions. Both types of microwires were demonstrated to completely crystallize on exposure to a temperature of more than 600 °C. During heating by a power up to 2.5 W (~400 °C), the microwires with Mo-Ni content were characterized by a monotonic resistivity increase with the temperature. But the microwires with Cr content had a well-marked resistivity minimum near the temperature Tм ~ 163 °C. This minimum shifted to a temperature value of ~260 °C after annealing. The research shows that the temperature dependence of resistivity in the temperature range below Tм obeys the f(T) = -ln(T) law.