The structural, electronic and magnetic properties of La0.55Ca0.45MnO3 were measured. A rapid change of lattice parameters appeared around 190 K associated with the az2 orbital ordering and charge ordering （CO） states that were reflected by both magnetization and resistivity. Great difference of magnetizations between the field-cooling （FC） and zero-field-cooling （ZFC） modes below the charge ordering temperature Too in high magnetic field （H〉4 T） was clearly seen. A field of 5 T （threshold field） is sufficient to completely destroy the antiferromagnetic （AFM） CO state for FC mode in magnetization while it is not the case for ZFC mode. A much larger field （larger than 10 T from ZFC resistivity data） is needed to destroy the CO state for ZFC mode. This could be explained by the coexistence and transformation of two phases reported by Huang et al. For ZFC mode, with increasing H, Tco gradually moves to low temperature regime and the relationship between the critical field Hc （0） to destroy CO state and Tco conforms to a power law Hc = Hc（0）（1 - T/Tco（0））^γ, where Hc （0） is the critical field to destroy the CO state at 0 K, and Too （0） is the CO temperature in zero field.
