The laser-heated diamond cell has been improved with the integration of in-situ X ray microprobe and double hot-plate heating techniques. A multimode YAG laser provides a flat-top power distribution at the focal spot. A hot-plate configuration is created where the heat generation and temperature measurement are concentrated at the planar interface of an opaque sample and transparent medium. The heating laser is split into two beams that pass through the opposed diamond anvils to heat the sample simultaneously from both sides. The temperatures of the two sides are measured separately with an imaging spectrograph and CCD and equalized by controlling the ratio of beam splitting. The axial temperature gradient in the sample layer is eliminated within the cavity of the two parallel hot plates. Uniform temperatures of 3000 (± 20) K have been achieved in high-pressure samples of 15-µm diameter × 10-µm thickness. X ray microprobe beam sizes down to 3.5 × 7 µm (i.e., significantly smaller than the laser heating spot) were used for in situ characterization of the samples under high P-T conditions. The technique has been used to study phase relations and melting of iron.