In this thesis, we study magnetic properties of Classical T Tauri Stars (CTTSs) using diffferent analysis methods from different angles. We first implement a least-squares deconvolution (LSD) code to enhance the signal-to-noise ratio of the observed data and therefore the ability to detect weak magnetic signatures in the polarized spectra. We compare our code to another LSD code by analyzing the same spectra with the same line list, and the good agreement confirms that our code performs similarly with other state of the art LSD codes. We have also tested the instrument we use to collect the spectropolarimetric data presented in this thesis. The intrument do produce spurious signals but at a very low level, posing no problem in our analysis. The polarization efficiency of the instrument is also high.

We then studied the CTTS BP Tau and found field configuration not consistent with other results in the literature, which may simply be due to the temporal variability given the observations were taken 8 years apart. Next, we detect extremely high levels of polarization on TW Hya in the 2013 epoch, which motivated us do a thorough magnetic variability study. We find that the field geometry does have changed, indicating a dynamo origin of these fields. We also try to use the rotational modulation of 6 CTTSs to locate the accretion spots. Some of the stars have shown definte change over the years while some remain similar.