Laser Interferometer Space Antenna ( LISA ) will routinely observe coalescences of supermassive black hole ( BH ) binaries up to very high redshifts . LISA can measure mass parameters of such coalescences to a relative accuracy of 10 ^ { -4 } -10 ^ { -6 } , for sources at a distance of 3 Gpc . The problem of parameter estimation of massive nonspinning binary black holes using post-Newtonian ( PN ) phasing formula is studied in the context of LISA . Specifically , the performance of the 3.5PN templates is contrasted against its 2PN counterpart using a waveform which is averaged over the LISA pattern functions . The improvement due to the higher order corrections to the phasing formula is examined by calculating the errors in the estimation of mass parameters at each order . The estimation of the mass parameters { \cal M } and \eta are significantly enhanced by using the 3.5PN waveform instead of the 2PN one . For an equal mass binary of 2 \times 10 ^ { 6 } M _ { \odot } at a luminosity distance of 3 Gpc , the improvement in chirp mass is \sim 11 \% and that of \eta is \sim 39 \% . Estimation of coalescence time t _ { c } worsens by 43 \% . The improvement is larger for the unequal mass binary mergers . These results are compared to the ones obtained using a non-pattern averaged waveform . The errors depend very much on the location and orientation of the source and general conclusions can not be drawn without performing Monte Carlo simulations . Finally the effect of the choice of the lower frequency cut-off for LISA on the parameter estimation is studied .