Micronized CaO with pores was synthesized by calcining the reaction product CaCO3from NH4HCO3and Ca(OH)2. Scanning electron microscopy, X‐ray diffraction, energy dispersive X‐ray spectroscopy, X‐ray fluorescence, Fourier transform infrared spectroscopy, and Brunauer–Emmett–Teller analysis were used to characterize CaO, which confirmed that after calcining at 800°C for 2 hr, CaCO3was completely converted into porous micronized CaO with a surface area of about 7.295 m2/g and a particle size of 0.5–1.5 μm. The porous CaO microparticles were used as heterogeneous catalysts for producing biodiesel from transesterification of soybean oil and methanol. The influences of reaction time, calcined temperature, and reusability of CaO were explored. The experiments showed that CaO has high catalytic activity for transesterification reaction, and the yield of biodiesel reaches more than 98% under the conditions of methanol/oil mole ratio of 9, and the catalyst amount (catalyst/oil) of 3% after reaction for 2.5 hr. The CaO catalyst can be recycled easily and it also has the advantage of low pollution. Simple synthetic route, low cost, high catalytic activity, good reusability, and great potential for industrialization are the advantages of the porous micronized CaO catalyst that was proposed in this work. Porous micronized CaO was easily prepared by calcining reaction product of NH4HCO3and Ca(OH)2, which presented a good activity and reusability in catalyzing transesterification reaction of oil with methanol for biodiesel. Simply synthetic route, low cost, high catalytic activity and great potential of industrialization production are the highlight attractiveness of the porous CaO microparticles, which was proposed in this work.