A thermographic non-destructive inspection method of a composite material structure, including an incidence surface and a bottom surface arranged at a distance from the incidence surface for detecting and measuring at least one volumetric defect that, starting from the incidence surface, extends towards the bottom surface, comprising the steps of: a- supplying a periodic thermal non-sinusoidal wave at the incidence surface; b- generating a temperature signal identifying a phase shift between the modulated thermal wave and a return thermal wave emitted from the structure in response to the modulated thermal wave, comprising generating a thermal map of at least one portion of the structure including the defect, the thermal map representing heat diffusive behaviour of regions, superficial and internal, of said at least one portion of the structure, wherein said phase shift is indicative of defect-free regions of the structure and of regions of the structure with defects; c- processing the temperature signal to obtain a first sub-signal related to the phase of the first harmonic of the temperature signal; d- identifying a first dimension of said defect as a function of the phase of the first harmonic of the temperature signal, said first dimension being the width of the defect; e- computing a first intermediate parameter (ΔΦ 1) by calculating the difference, in degrees, between a minimum value and a maximum value, or between a minimum value and a mean value of a plurality of values in a neighbourhood of the maximum value, of the first sub-signal; f- processing the temperature signal to obtain a second sub-signal related to the phase of the third harmonic of the temperature signal; g- calculating a second intermediate parameter (ΔΦ 2) as the difference, in degrees, between a minimum value and a maximum value, or mean of a plurality of values in a neighbourhood of the maximum value, of the second sub-signal; h- identifying a second dimension of the defect as a function of the first dimension, the first intermediate parameter (ΔΦ 1) and the second intermediate parameter (ΔΦ 2). a- supplying a periodic thermal non-sinusoidal wave at the incidence surface; b- generating a temperature signal identifying a phase shift between the modulated thermal wave and a return thermal wave emitted from the structure in response to the modulated thermal wave, comprising generating a thermal map of at least one portion of the structure including the defect, the thermal map representing heat diffusive behaviour of regions, superficial and internal, of said at least one portion of the structure, wherein said phase shift is indicative of defect-free regions of the structure and of regions of the structure with defects; c- processing the temperature signal to obtain a first sub-signal related to the phase of the first harmonic of the temperature signal; d- identifying a first dimension of said defect as a function of the phase of the first harmonic of the temperature signal, said first dimension being the width of the defect; e- computing a first intermediate parameter (ΔΦ 1) by calculating the difference, in degrees, between a minimum value and a maximum value, or between a minimum value and a mean value of a plurality of values in a neighbourhood of the maximum value, of the first sub-signal; f- processing the temperature signal to obtain a second sub-signal related to the phase of the third harmonic of the temperature signal; g- calculating a second intermediate parameter (ΔΦ 2) as the difference, in degrees, between a minimum value and a maximum value, or mean of a plurality of values in a neighbourhood of the maximum value, of the second sub-signal; h- identifying a second dimension of the defect as a function of the first dimension, the first intermediate parameter (ΔΦ 1) and the second intermediate parameter (ΔΦ 2).