Vibration Analysis by using FFT I have 6 column vectors $(\mathbf A_x,\mathbf A_y, \mathbf A_z, \mathbf G_x, \mathbf G_y, \mathbf G_z)$ taken by accelerometer to analyze the vibration of a construction. Each column is a $64998\times 1$ valued-vector and overall time is $664$ seconds. I've done something but I don't know how I can fix it. 

Fs=97.885;
L=64998;
NFFT=2^nextpow2(64998);

AXdft = fft(Ax);
AXdft=AXdft(1:NFFT/2+1,:);

freq = linspace(0,Fs,NFFT/2+1);

plot(freq,2*abs(AXdft));
xlabel('Hz');ylabel('Magnitude');

figure;
plot(freq,20*log10(abs(AXdft)));
xlabel('Hz');ylabel('dB');
title('Plot in dB');


BTW I will add each column over other via hold on

getting a figure but missing its size. The frequency doesn't appear in figure and I guess I must do PSD, Can anyone help?
I believe that your problem is using nextpow2 to set the fft length.  fft(x) returns a vector length(x). fft(x) is not constrained to a power of 2. If you want to set a particular size by zero padding, use fft(x,N). NFFT=length(Ax) is the actual size of DFT.

Using psd has advantages but if you use the proper size, your code will work (assuming I am correct) . Taking a single long DFT isn't an accepted way to estimate a psd, unless you are looking for very fine, high snr features.