In the production process of ultra-high strength steel, heating schedule of casting slab is one of primary controlling parameters in hot rolling techniques. Heating temperature and holding time affect the prior austenite grain size and the solution of microalloyed elements directly, which both affect austenite recrystallization, precipitation and mechanical properties. A plenty of researches have been made to get better understanding and controlling the austenite grain size or precipitation behavior during austenitizing process over the past half a century, but it lacks systematic researches. Hence it is very important to confirm a reasonable heating schedule. In this work, the austenite grain coarsening behavior and microalloying carbonitrides dissolving behavior in 1000 MPa grade Nb-Ti microalloyed ultra-high strength steel during isothermal holding at different temperatures were studied by OM, TEM and EDS. The results showed that the precipitates in the slab can be obviously classified into three kinds by their size and shape. The average dimension of the bigger cubic precipitates is over 1 μm, and that of the smaller spherical, ellipsoid or cubic precipitates is below 500 nm or less. EDS results showed that the bigger cubic precipitates are TiN, and the smaller spherical, ellipsoid or cubic precipitates are mainly composed of Nb, Ti composite precipitates and a bit of TiS or Ti (C, S). With the increasing of holding temperature, increase of the original austenite grain size showed a monotonous increase trend, and the austenite grain grows rapidly when the heating temperature exceeds 1200 ℃; the amount of precipitates decreased and their size increased, the atomic ratio of Ti/Nb increased gradually, and EDS results showed all the precipitates contain Nb, Ti elements. With the increasing of holding time, the average austenite grain grew up in parabolic law, the amount of small sized spherical and ellipsoid precipitates dissolved gradually, and that of the large sized cubic precipitates increased gradually and their edges become blurred. The effects of heating temperature and holding time on austenite grain size and precipitation behavior were considered synthetically, the heating temperature at 1250 ℃ by holding 80 min will be more appropriate for the 1000 MPa grade Nb, Ti microalloyed ultra-high strength steel.