Tip, substrate, molecular species, and laser are the four main factors determining the detection sensitivity and spatial resolution of tip-enhanced Raman spectroscopy (TERS). In this paper, we deal with the tip and molecular species. We will first introduce a simple method to regenerate Au tips fabricated by electrochemical etching for TIERS to ensure good experimental reproducibility. Some criteria are proposed to choose molecular species as the probe molecules for reliable TERS investigation of the surface processes and TERS mechanism after a comparison of the TERS and SERS signal of benzenedithiol and 4,4'-bipyridine. Then, high-quality TERS signals of a series of molecules of oligo(phenylene ethynylene)s (OPEs) of different lengths adsorbed on a Au(111) single crystal surface are demonstrated. The possible explanation assisted by finite-difference time-domain (FDTD) simulation and quantum chemical calculation is used to understand the molecular length-dependent change of the TERS signal. The potential application of TERS for studying systems related to molecular electronics is proposed. Finally, a method is proposed to calculate the tip enhancement for the nonresonant surface species in TERS. Copyright (C) 2009 John Wiley & Sons, Ltd. ; National Basic Research Program of China [2009CB930703, 2007CB935603]; Natural Science Foundation of China [20673086, 20825313, 20827003]; Ministry of Education of China [NCET-05-0564]; Fok Ying Tung Foundation [101015]