We compute the kinematical dependence of the peak luminosity, the pulse width and the spectral lag of the peak luminosity on the viewing angle θv of a jet. For appropriate model parameters we obtain the peak luminosity-spectral lag relation similar to the observed one including GRB980425. A bright (dim) peak with short (long) spectral lag corresponds to a jet with small (large) viewing angle. This suggests that the viewing angle of the jet might cause various relations in GRBs such as the peak luminosity-variability relation and the luminosity-width relation. Our model also suggests that X-ray rich GRBs (or X-ray flushes or Fast X-ray transients) are typical GRBs observed from large θv with large spectral lag and low variability
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