Here is a small example where a and b are padded with zeros, so that regular convolution can be compared with convolution by fft and ifft. (The zeros ensure that for fft and ifft, which do circular convolution, the nonzero parts can't overlap by going 'the other way around the circle.'
a = [(1:3)+i*(2:4) zeros(1,4)]
b = [(3:5)+i*(4:6) zeros(1,4)]
n = length(a)
convab = conv(a,b)
convab1 = ifft(fft(a).*fft(b))
convab2 = n*fft(ifft(a).*ifft(b))
All of these results agree (not counting that the conv result is a longer vector and contains more zeros than the other two). For the convab2 result you have to multiply by an extra factor of n. This is because the Matlab ifft algorithm contains an overall factor of (1/n) and the fft does not.